Previous results on the potentiation by cytochalasin B (CB) of lymphocte responses in vitro, particularly the requirement for serum for this potentiation, prompted us to undertake the present study on inhibitory effects of autologous serum on rat lymphocyte activities. Main results mirror the inverse effect of CB: maximal inhibition is obtained at suboptimal doses of PHA; inhibitory effect bears essentially on early events of activation; high cell density is of prime importance. In contrast to CB, serum affects more 3H-TdR incorporation than blast transformation. Therefore, CB and some serum component(s) could have inverse effects on a single regulatory mechanism: suppressive cellular interactions.