Aroclor 1254 and twelve pesticides of different chemical classes were tested for their in vitro effects on lymphocyte mitogenic responses to PHA, rosette formation with sheep red blood cells (E-rosette) and neutrophil chemotaxis. By using a diluted whole blood culture, butoxide at 10 micrometers inhibit lymphocyte mitogenic response to PHA by 26%. Inhibition by other pesticides were less than 20% (5--20(). Higher degrees of inhibition were observed if the lymphocytes were pretreated with pesticides in the absence of serum: pp-DDT inhibited by 73% trithion by 50% and benzylthiocyanate by 30%. E-rosette formation was less inhibited by these pesticides except DDT, which inhibited the E-rosette formation by 50% at 10(-4)M. Only methyl parathion was found to have some inhibitory effects on neutrophil chemotaxis. These results indicate that insecticides may have direct effects on the leukocyte function and that the presence of serum may alter the interaction of insecticides and leukocyte membranes.