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The effect of high doses of PHA in autoimmune thyroiditis was studied. The autoimmune response was elicited by the injection of syngeneic thyroglobulin (Tg) followed 3 hr later by LPS on days 0 and 7. PHA was injected 48 and 24 hr before the first antigen administration. The antibody titer to thyroglobulin and the degree of cellular infiltration in the thyroid were determined thereafter. It was found that PHA reduces the humoral response to Tg and completely suppresses the infiltration in the thyroid. However, this suppression is no longer observed 7 weeks later. By testing this effect with CFA instead of LPS, no PHA-induced suppressive effect was seen. The highest suppression is found when PHA is given close to the antigen injection.