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An extract from calf spleens, injected into mice, was found to inhibit their lymphocyte proliferative response to PHA-M and PWM in vitro. Despite the ability of the spleen extract to inhibit the response of lymphocytes to stimuli in vitro, no effect was observed on the repopulation of lymphocytes in the peripheral blood of sublethally irradiated mice. The data suggest that the spleen extract acts as a specific inhibitor of the innume competent cells since neither the precursors of lymphocytes nor other haematopoietic cells were affected.