Cyclic AMP phosphodiesterase in human lymphocytes and lymphoblasts.

Cyclic nucleotide phosphodiesterase activities were examined in lymphocytes from 12 transformed human B cell lines, two T cell lines, six patients with lymphocytic leukemia, and 10 normal donors. A consistent difference bwtween cells from the normal and leukemic state was observed. The cyclic AMP phosphodiesterase activity from normal lymphocytes is inhibited greater than 80% by muM cyclic GMP while this concentration of nucleotide has little or no effect on the enzyme from transformed lymphocytic cell lines or from lymphocytic cells of leukemia patients. The reported lack of cyclic GMP phosphodiesterase in human lymphocytes from several sources is confirmed. The apparent absence of a cyclic GMP degradation mechanism and of cyclic GMP control of cyclic AMP hydrolysis may be related to defective lymphocyte growth control.     

Characteristics of ribosomal RNA synthesis in the blood lymphocytes of chronic lympholeukemia patients

A study was made of the number of silver grains over the nucleoli and of the content of ribosomes in the lymphocyte cytoplasm for six healthy persons and for 20 patients with chronic lymphatic leukemia. Besides ratios of compact, nucleolonemic and ring types of nucleoli were evaluated in addition to counts of the specific radioactivity of mature 28S rRNA in lymphocytes. In the majority of cases examined, cells with 1 or 2 nucleoli of compact and nucleolonemic types were seen dominating. The number of silver grains over the nucleoli in the control healthy persons did not differ from that in patients who did not receive any treatment, which contrasted with high value grain counts in the treated patients. The lymphocyte ribosome contents varied within the normal and decreased values in both the patient groups. The specific radioactivity in 28S rRNA leukemic lymphocytes was significantly lower in groups of patients with low ribosome contents than in those with the normal ones. The data suggest that in the leukemic cells with a high or unaltered activity of ribosome cistrons and low ribosome levels rRNA processing is broken.     

Human lymphocyte tubulin: Purification and characterization in normal and leukemic cells

Tubulin has been purified from human blood and tonsil lymphocytes. Using gel filtration, the molecular weight of human lymphocyte tubulin was estimated to be 119 000. The proteins was shown to consist of two subunits, with molecular weights of 61 000 and 58 000 comparable to the α and β polypeptides of human brain tubulin. A partial identity reaction was observed between lymphocyte tubulin and human tubulin when tested by double immunodiffusion against a rabbit anti-human brain tubulin antibody. In the presence of GTP, the purified protein polymerized to form microtubules. Tubulin was localized to the cell's juxtacentriolar region by immunofluorescence and electron microscopy. When assayed by a colchicine-binding assay corrected for time decay, the binding affinity was 1.50 ± 0.86 · 10⁶M^?1 and a level in normal lymphocytes of 1.21 · 10^?2 ± 0.79 g/g of soluble protein was determined. Since chronic lymphocytic leukemia lymphocytes have an anomalous capping behavior as well as an unusual susceptibility to colchicine toxicity, the properties and levels of tubulin were determined in these cells. Similar values were obtained for the level, decay rate, molecular weight, and K_a for colchicine as for normal lymphocytes. Chronic lymphocytic leukemia lymphocyte tubulin polymerized in a normal fashion. It thus appears that a decrease in the quantity or function of tubulin does not account for these anomalies in the chronic lymphocytic leukemia lymphocyte.     

Protein A-binding immunosuppressive mouse serum factors

Summary In this study protein A of Staphylococcus aureus has been used to isolate an immunosuppressive component present in mouse serum. The suppressive effect of mouse serum on lymphocyte activation was partially abrogated by prior adsorption on protein A, and also by ammonium sulfate precipitation or specific immune precipitation with anti-IgG but not with anti-IgM. Protein A-binding material was isolated after chromatography on protein A-Sepharose and studied in spleen cell cultures. Protein A eluates from normal or NZB/NZW mice were found to suppress concanavalin A (Con A)-activated normal mouse spleen cells, and suppression was more potent with NZB/NZW serum isolates. Suppressive activity was dependent upon the dose of eluate added to cell cultures. The suppressive effect of NZB/NZW protein A-binding material was apparent in both Con A- and lipopolysaccharide (LPS)-stimulated normal mouse spleen cells, and required early addition to the cell cultures or preincubation with target lymphocytes. The suppressive activity was not detectably cytotoxic during a suppressive preincubation period. The possible relevance of these observations to experimental strategies in tumor immunotherapy is discussed.     

Graft-versus-host response,measured by splenomegaly assay,by populations of allosensitized mouse lymphocytes

The graft-vs-host (G-v-H) reactivity of sensitized or nonsensitized mouse lymphoid cell populations was measured using a splenomegaly assay. Sensitized populations were obtained either from the local lymph nodes of alloimmunized animals or from the spleens of heavily irradiated mice previously infused iv with allogeneic lymphocytes (educated cells). Immunization of animals resulted in increased G-v-H responses of the cells in their local lymph nodes. This effect was more pronounced when the immunizing cells differed only at non-H-2 transplantation antigens than when H-2-disparate strain combinations were tested. There was no evidence of a changed doseresponse profile of lymphocytes obtained from immunized mice. The G-v-H reactivity of educated cell populations was complex. The slopes of the dose-response lines obtained for lymph node cells or thymic cells educated in an H-2-disparate strain were generally lower than those obtained for nonsensitized cells. This difference was particularly evident when testing educated thymocytes. By studying the G-v-H indices obtained in A/Sn × C57B1 hybrids after inoculation of nonsensitized C57B1 lymph node cells or specifically educated C57BL lymph node cells, it was observed that the latter cells were approximately 30 times more reactive when small cell inocula were compared. On the contrary, education of lymphocytes in H-2-compatible allogeneic hosts did not result in any increment of their G-v-H reactivity. The results indicate that different methods of sensitizing lymphocyte populations against alloantigens may lead to activation of different subclasses of T-cells which differ in their mode of antigen reactivity.     

Immortalization of human lymphocytes by fusion with cytoplasts of transformed mouse L cells

Fusion of mouse L929 cytoplasts with human peripheral blood lymphocytes induced lymphocyte proliferation that gave rise to lymphoid cell lines of B and T cell origin with unlimited growth potential. The immortalized cell lines were routinely grown in standard medium supplemented with fetal calf serum. Furthermore these cell lines could be propagated in chemically defined serum-free media. Each establishment of lymphoid cell lines was preceded by a proliferation phase 2 wk after cytoplast/cell fusion, which appears to be a necessary step in the immortalization process. The immortalized cells have a nearly normal human karyotype, do not form colonies in soft agar medium, and are not tumorigenic in nude mice. Cloned B cell lines produced human immunoglobulins of heavy and light chain types. No cross-reaction with DNA of herpes simplex virus, human cytomegalovirus, human T cell leukemia/lymphoma virus I and II, or polyoma virus was detected in the genome of immortalized cell lines by Southern blot hybridization. Furthermore B and T cell lines were established that appear to be free of Epstein-Barr virus genome.     

Regulated expression of the RB "tumor suppressor gene" in normal lymphocyte mitogenesis: elevated expression in transformed leukocytes and role as a "status quo" gene.

Expression of the RB retinoblastoma tumor suppressor gene product is regulated early during the stimulation of normal human peripheral blood lymphocytes, suggesting a regulatory role for the amount of this protein in mitogenesis of normal cells. When normal human peripheral blood lymphocytes were mitogenically stimulated with pokeweed mitogen, bivariate flow cytometric measurements of cellular DNA and RB protein content showed an early decrease in the amount of RB protein per cell, anteceding onset of S phase. A subsequent increase in the amount of RB protein per cell occurred with cell proliferation. Thus the amount of RB protein relative to the total cell mass underwent a biphasic response with mitogenesis. The resulting proliferating cells had a slightly elevated level of RB protein per cell compared to the unstimulated cells. Comparison of other proliferating leukocytes to normal lymphocytes showed that both EBV virally transformed lymphocytes and human promyelocytic leukemia cells (HL-60) had elevated levels of RB protein per cell compared to normal peripheral blood lymphocytes. Mitogenic stimulation or transformation by other means thus is associated with regulation of the amount of RB protein per cell, suggesting a regulatory role for the RB protein in normal cell growth control.     

B lymphocyte production in the bone marrow of mice with X-linked immunodeficiency (xid)

CBA/N mice carry an X-linked recessive immunodeficiency (xid) gene manifested by the absence of a B lymphocyte subpopulation, but the manner in which the xid gene exerts its effect on B lymphocyte development is unknown. The production of B lymphocytes in the bone marrow of CBA/N mice has now been compared with that of normal CBA/J mice by using two in vivo assays: immunofluorescence stathmokinetic studies measured pre-B cell proliferation, whereas radioautographic [3H]thymidine labeling was used to evaluate small lymphocyte turnover. Although the total cellularity of CBA/N mouse bone marrow was greater than normal, the absolute number of marrow small lymphocytes, pre-B cells, and B lymphocytes were all similar to those in CBA/J controls. Furthermore, in the bone marrow of CBA/N mice, the proliferation rate of pre-B cells, calculated from their rate of entry into mitosis, and the turnover rate of small lymphocytes, derived from their rate of [3H]thymidine labeling, were not significantly different from those seen in nondefective mice. The present findings that pre-B cell proliferation and small lymphocyte production proceed at similar rates in the bone marrow of xid and normal mice suggest that the xid gene does not act at the level of primary B cell genesis in the bone marrow. The findings are in accord with the view that the xid gene produces a maturation block or a functional abnormality among B lymphocytes in the peripheral lymphoid tissues rather than the deletion of a sublineage of B lymphocytes in the bone marrow.     

A human B lymphocyte antigen (P-76) shared by B-cell chronic lymphocytic leukemia cells and hairy cell leukemia cells

A membrane antigen with an apparent specificity to B lymphocytes was detected with immunochemical techniques and its properties were analyzed. Anti-B-CLL serum was raised in a rabbit by immunization with B-cell chronic lymphocytic leukemia (B-CLL) cells. This anti-B-CLL serum was absorbed with erythrocytes, liver homogenate and insolubilized immunoglobulins. After further absorption with T-CLL cells, chronic myelocytic leukemia (CML) cells and acute myelocytic leukemia (AML) cells, the anti-B-CLL serum still reacted with peripheral blood B lymphocytes, B-CLL cells and hairy cell leukemia (HCL) cells. In contrast, no reactivity was seen with peripheral blood T lymphocyte or monocytes, or leukemia cells of non-B cell origin. An immunoprecipitation of radiolabeled cell surface proteins was attempted using the anti-B-CLL serum in the presence of Staphylococcus Aureus Cowan 1 (SaCl), and the precipitates were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). A membrane antigen with an apparent molecular weight of 76,000 daltons (P-76) was immunoprecipitated with the anti-B-CLL serum from the lysates of normal B lymphocyte, B-CLL cells and HCL cells. The antigen (P-76) is not composed of disulfide-linked subunits and has no structural relationship with HLA-DR (Ia-like) antigens or other known antigens. These results suggest that this antigen is B-lymphocyte specific, and favour the B-lymphocyte nature of HCL cells.     

Reduced cell turnover in bovine leukemia virus-infected, persistently lymphocytotic cattle

Although nucleotide analogs like bromodeoxyuridine have been extensively used to estimate cell proliferation in vivo, precise dynamic parameters are scarce essentially because of the lack of adequate mathematical models. Besides recent developments on T cell dynamics, the turnover rates of B lymphocytes are largely unknown particularly in the context of a virally induced pathological disorder. Here, we aim to resolve this issue by determining the rates of cell proliferation and death during the chronic stage of the bovine leukemia virus (BLV) infection, called bovine persistent lymphocytosis (PL). Our methodology is based on direct intravenous injection of bromodeoxyuridine in association with subsequent flow cytometry. By this in vivo approach, we show that the death rate of PL B lymphocytes is significantly reduced (average death rate, 0.057 day(-1) versus 0.156 day(-1) in the asymptomatic controls). Concomitantly, proliferation of the PL cells is also significantly restricted compared to the controls (average proliferation rate, 0.0046 day(-1) versus 0.0085 day(-1)). We conclude that bovine PL is characterized by a decreased cell turnover resulting both from a reduction of cell death and an overall impairment of proliferation. The cell dynamic parameters differ from those measured in sheep, an experimental model for BLV infection. Finally, cells expressing p24 major capsid protein ex vivo were not BrdU positive, suggesting an immune selection against proliferating virus-positive lymphocytes. Based on a comparative leukemia approach, these observations might help to understand cell dynamics during other lymphoproliferative disease such as chronic lymphocytic leukemia or human T-cell lymphotropic virus-induced adult T-cell leukemia in humans.     

Chronic lymphocytic leukemia lymphocytes: membrane anomalies and H2O2 vulnerability

This paper reviews selected biochemical and functional studies characterizing B lymphocytes from patients with chronic lymphocytic leukemia. When compared to B lymphocytes from the circulation of normal subjects, a number of differences are noted. Functionally, the CLL B lymphocyte is impaired in mitogen response, cap formation following attachment of multivalent ligands, and the density of surface immunoglobulins. It also differs from normal in the content of the ectoenzyme 5' nucleotidase, which is often decreased, and the concentration of ascorbic and dehydroascorbic acid, which are markedly elevated in these cells. The level of tocopherol is decreased. CLL and normal B lymphocytes are more vulnerable than T lymphocytes to the toxic effect of H2O2. This sensitivity is Ca2+ dependent and inhibited by nanomolar concentrations of nonsteroidal antiinflammatory agents. These studies identify the human B lymphocyte as a cell that should be a suitable target for selective killing by H2O2.     

Tolerance to class II major histocompatibility complex molecules is maintained in the presence of endogenous, interleukin-2-producing, tolerogen-specific T lymphocytes

We have examined the requirement for clonal reductions of tolerogen-reactive lymphocytes in mice of the A strain background rendered neonatally tolerant of class II major histocompatibility complex molecules. Tolerogen-specific mixed lymphocyte reactivity of lymphocytes obtained from 130 adult, class II tolerant mice, bearing a healthy skin allograft, was examined. Lymphocytes obtained from 86 mice responded to the tolerogen, in vitro, with a positive mixed lymphocyte response (MLR) indicating that a large proportion (75%) of adult class II tolerant mice on the A strain background are not clonally deleted for tolerogen-reactive lymphocytes. In addition, lymphocytes from 29 mice were MLR-negative to the tolerogen, and lymphocytes from 15 mice demonstrated such high amounts of proliferation to syngeneic stimulators that their specific response to the tolerogen could not be determined. In view of the discordance between the in vivo and in vitro expressions of tolerance in the MLR-positive mice, lymphocytes from these mice were compared with normal lymphocytes by several assays. 1) Tolerogen-specific proliferative responses obtained from both normal and tolerant lymphocytes could be inhibited by the addition of monoclonal antibodies specific for the relevant class II antigens; 2) quantitative differences in the ability of normal, as compared with tolerant cells, to respond to the tolerogen in the MLR were not apparent; 3) no evidence of qualitative differences in the cell-surface phenotype of the proliferating cell was observed, (i.e., the cells were Thy-1+, L3T4+, Lyt-2-); and 4) lymphocytes from both normal and MLR-positive tolerant mice produced substantial amounts of interleukin-2 in response to the tolerogen. Thus, clonal deletion of helper cells is not required for tolerance to class II major histocompatibility complex antigens and we propose that tolerance may be maintained by either 1) in vivo suppression of the tolerogen-specific helper cells or 2) selective deletion or suppression of class II specific effector cells.     

T lymphocyte tolerance and early appearance of virus-induced cell surface antigens in Moloney-murine leukemia virus neonatally injected mice

Regression of Moloney-murine sarcoma virus- (M-MSV) induced sarcomas in normal adult mice is accompanied by generation of virus-specific cytotoxic T lymphocytes (CTL). However, when neonatal mice that were injected with Moloney-murine leukemia virus (M-MuLV carrier) were subsequently challenged as adults with M-MSV, the sarcomas did not regress nor did they generate CTL. This failure to produce CTL cannot be ascribed to nonspecific immunodepressive effects or to suppressor cell generation since M-MuLV carrier mice exhibit normal reactivity after allogeneic cell stimulation. Moreover, addition of M-MuLV-infected cells as the third party to cultures does not reduce activity of CTL from M-MSV immune mice. Since M-MSV and M-MuLV possess common antigens, the observed unresponsiveness was considered in relationship to induction of a T lymphocyte tolerance, which may follow introduction of foreign antigens at an early stage of development. In fact, it was observed that as early as 10 days after injection, thymus, lymph node, and spleen from M-MuLV carrier mice express virus-induced cell-surface antigens that not only are targets for M-MSV-immune CTL, but also induce in vitro a strong specific cytotoxic response. In addition, a cold target inhibition assay disclosed that the same antigens are shared by both M-MuLV infected and leukemia cells, even though they are less expressed on the surface of the former. The finding that the cytotoxicity of alloreactive lymphocytes from M-MuLV carrier mice is reduced after preincubation with M-MSV immune CTL confirms that virus infection does not bring about functional inactivation of lymphocytes. Finally, it was observed that virus antigen presence on lymphocytes from M-MuLV neonatally injected mice is closely related to subsequent leukemia development.     

Ferritin-bearing lymphocytes and T-cell levels in peripheral blood of patients with breast cancer

Summary Peripheral blood lymphocytes bearing surface ferritin and thymus-dependent lymphocyte (T cell) levels were determined in 15 breast cancer patients in stage I–II, 5 in stage III, 10 with benign breast disease, 4 with Thalassaemia, and 25 normal controls. The results of this study demonstrate that a subpopulation of lymphocytes (16.6%) bearing surface ferritin was found in patients with breast cancer in stage I–II. None were demonstrated in patients with either benign breast disease, or with Thalassaemia, the latter known to have high serum ferritin levels, and almost none (1.7%) in normal individuals. A significant decrease in the percentage of ERFC as compared with the percentage of T cells, determined with anti-T cell antiserum (P<0.01), was observed in patients with breast cancer in stage I–II. Yet, the mean T-cell percentage in this group of patients was significantly higher than the mean percentage of T cells in normal controls (P<0.01). In patients with benign breast disease, the percentage of T cells corresponded to the percentage of ERFC and did not significantly differ from those in normals. Stage III breast cancer patients seem to constitute a biologically distinct group, since the ferritin-positive lymphocyte subpopulation disappeared and the percentage of ERFC and T cells returned to the values of normal controls.Overnight incubation of lymphocytes from patients exhibiting a ferritin-positive lymphocyte subpopulation in culture media containing 20% FCS resulted in the removal of ferritin from the surface of the cells and in restoration of the percentage of ERFC.     

Age-related changes in LFA-1 expression, cell adhesion, and PHA-induced proliferation by lymphocytes from senescence-accelerated mouse (SAM)-P/8 and SAM-R/1 substrains.

Accelerated senescence-prone mice of the SAM-P/8jf series were compared with senescence-resistant SAM-R/1 controls in terms of age-related changes in phytohemagglutinin (PHA) proliferative responses and lymphocyte function-associated antigen-1 (LFA-1) utilization by non-adherent splenocytes. Advancing age was associated with a reduction in cell proliferative responses to PHA in both substrains, although the rate of decline was significantly more rapid in the senescence-prone animals. Conversely, in both substrains there was a progressive age-related increase in the proportion of splenocytes expressing high levels of LFA-1, and a parallel increase in the degree of LFA-1-dependent cell aggregation induced by phorbol ester. Age-matched SAM-P/8jf and SAM-R/1 mice did not differ in terms of LFA-1 expression or LFA-1-dependent cell aggregation. Two-color cytofluorometric analysis demonstrated the enhanced expression of LFA-1 expression by cells bearing the Pgp-1hi phenotype characteristic of memory lymphocytes. These results suggest that age-associated changes in lymphocyte adhesion are attributable to alterations in the relative numbers of memory cells expressing high levels of LFA-1, but are unlikely to contribute to the reduced proliferative response to mitogen in aged mice.     

运动员剧烈运动后血中应激免疫抑制蛋白的产生

我们曾经报道,大鼠或小鼠在束缚应激后血中产生了一种能抑制免疫功能的应激免疫抑制蛋白,（又称Ｎｅｕ－ｒｏｉｍｍｕｎｅｐｒｏｔｅｉｎ,ＮＩＰ,神经免疫蛋白）。本工作证明,运动员在大运动量的训练后血清中也产生一种能抑制淋巴细胞转化的物质,它的生化特性及分子量与前述大鼠和小鼠中的应激免疫抑制蛋白相同。在体外实验中,应激大鼠的血清培养人淋巴结细胞,获得了与大鼠实验相同的结果,即人淋巴结细胞也能产生应激免疫抑制蛋白。同时小鼠束缚应激的血清和大运动量的人类血清可以分别抑制人正常淋巴细胞和正常小鼠由ＣｏｎＡ诱导的淋巴细胞转化,以上结果表明,这种应激免疫抑制蛋白的种属特异性不强。     

Identification of fibronectin receptors on T lymphocytes

We report the identification of fibronectin receptors on thymocytes and T lymphoma cells. Affinity chromatography of extracts of the T cell lymphoma, WR16.1, on a fibronectin-Sepharose column combined with specific elution using a synthetic peptide containing the cell attachment-promoting sequence, arginine-glycine-aspartic acid, yielded two polypeptide components having apparent molecular masses of approximately 160 kD reduced and 175 and 150 kD nonreduced. Immunoprecipitations from surface-iodinated WR16.1 cells or fibronectin-adherent thymocytes using a rabbit antiserum raised against the fibronectin receptor that is present on human fibroblasts revealed, in each case, the same two radiolabeled components. In contrast, immunoprecipitation from fibronectin-nonadherent T lymphoma cells, designated WR2.3, revealed the presence of only the smaller subunit. Although the lymphocyte receptor and the fibronectin receptor identified on fibroblasts share immunologic determinants, they differ in that the molecular mass of the lymphocyte protein is larger. Moreover, trypsinization of either thymocytes or the WR16.1 T lymphoma cells resulted in a subsequent loss of their ability to adhere to fibronectin-coated substrates and a reduction in the electrophoretic mobility of each of the polypeptide chains of the fibronectin receptor present on their surfaces. These changes, however, were not observed with normal rat kidney fibroblasts or mouse 3T3 fibroblasts in response to trypsinization. The data establish the existence on normal lymphocytes of fibronectin receptors that are quite similar to those found on fibroblasts. The possible function of this molecule on thymocytes is discussed.     

Cortisol catabolism by lymphocytes of patients with chronic lymphocytic leukemia

A low rate of catabolism of cortisol by lymphocytes correlates with high sensitivity of the cells to the steroid and causes them to die at a greater rate than control samples. Since lymphocytes of patients with chronic lymphocytic leukemia respond to treatment with glucocorticosteroids and are cortisol sensitive, we attempted to see whether their capability to catabolize cortisol differs from that of normal lymphocytes. No difference was found between the two groups of cells with regard to the pattern of cortisol metabolites. However, the lymphocytes of the chronic lymphocytic leukemia groups showed a total cortisol catabolism per cell that was significantly lower than that of the control group. Patients with low lymphocyte count in peripheral blood showed a relatively higher cortisol metabolism by lymphocytes per cell than those with high counts.     

Interaction of blood lymphocyte Fc receptors with IgG during in vitro cultivation

Bovine blood lymphocytes taken from normal cows and those suffering from chronic lymphocytic leukemia were cultured in complete medium 199 with 10% of heat-inactivated fetal bovine serum. After a 48 hour culturing an enhanced quantity of the Fc-receptor bound fluoresceinated immunoglobulin G (IgG) was established. When lymphocyte fractions enriched with T- or B-cells were cultured, the binding capacity of Fc-receptors for IgG (48 hours after culture) increased in both the cell populations. A study of the kinetics of interactions of Fc-receptors with IgG showed that the increased number of Fc-receptors after culturing was followed by an enhanced affinity of Fc-receptors towards IgG. The affinity of Fc-receptors of blood lymphocytes of cows with chronic lymphocytic leukemia was lower than that of normal lymphocytes.     

Differentiation of lymphocytes in mouse bone marrow. I. Quantitative radioautographic studies of antiglobulin binding by lymphocytes in bone marrow and lymphoid tissues

The density of surface immunoglobulin on small lymphocytes in the bone marrow and other lymphoid tissues has been compared by radioautographic measurements of antiglobulin binding.Cell suspensions from CBA mice were exposed to ¹²⁵I-labeled rabbit anti-mouse globulin in a wide range of concentrations for 30 min at 0 °C. With increasing concentration of antiglobulin-¹²⁵I the percentage of labeled antiglobulin-binding small lymphocytes in spleen and lymph node suspensions reached well-defined plateau levels. Very few normal or cortisone-resistant thymus cells were labeled under identical conditions. Bone marrow small lymphocytes showed a linear increment in labeled cells throughout the antiglobulin-¹²⁵I dose range, their labeling intensity varied widely, and approximately one half remained unlabeled at high antiglobulin-¹²⁵I concentrations. In 6 wk-old congenitally athymic mice the bone marrow small lymphocyte labeling pattern resembled that in CBA mice, while nearly all (91–97%) small lymphocytes in lymph nodes, thoracic duct lymph and blood, and 75% of those in the spleen, became labeled under plateau conditions. Treatment of cells from 10 wk-old CBA mice with AKR anti-θ C3H serum and complement resulted in almost complete (93%) antiglobulin-labeling of residual small lymphocytes from the spleen but had little effect on bone marrow lymphocyte labeling. Under germfree conditions the proportion of antiglobulin-binding small lymphocytes was slightly elevated in all lymphoid tissues of CBA mice.The results demonstrate that many of the small lymphocytes in mouse bone marrow have readily detectable surface immunoglobulin molecules which vary considerably in density from cell to cell, while others neither have detectable surface immunoglobulin, nor are they θ-bearing, thymus-dependent or recirculating cells. The concept of bone marrow small lymphocytes as a maturing cell population is discussed.