Abstract: | When seeded in small numbers in medium containing 10?6M aminopterin and fetal calf serum, V79 Chinese hamster cells required dialyzable components from the serum for growth. However, the cells grew in medium containing 10?6M aminopterin and dialyzed serum, provided that the medium was supplemented with 10?5M hypoxanthine and sufficient 5·10?6M) thymidine. A growth-inhibitory property of some batches of dialyzed serum was abolished on heating the serum for 30 min at 56°. Three lines of V79 cells which lacked detectable hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity were seleccted in medium containing 8-azaguanine (8-AzG). In two of these, no spontaneous reversion to the HGPRT+ phenotype was detectable, and these cells did not cooperate metabolically with HGPRT+ cells to prevent the growth of the latter in HAT medium. One of the HGPRT? lines showed a high rate of spontaneous reversion (118/105 cells) in medium containing undialyzed serum. However, in medium containing dialyzed serum the spontaneous reversion rate fell to , suggesting that the revertants arising in medium containing undialyzed serum were biochemically heterogeneous. |