Malathion has been previously shown to cause allergic responses and suppress the generation of a humoral immune response in vivo. In this study, the effect of in vivo administration of malathion on cellular, humoral and mitogenic responses was examined. Acute (50% LD 50) or subacute (10% LD 50 per day for 14 days) treatment with malathion in vivo did not affect the in vivo generation of specific antibody secreting cells to sheep red blood cells (SRBC) or cytotoxic T lymphocytes (CTL) to allogeneic tumor. However, 5 days following acute administration of malathion, there was a slight increase in humoral immune responsiveness. Acute treatment with 50% LD 50 purified malathion did not affect body weight, splenic cell number, or thymus size. However, mitogenic responses to Concanavalin A (Con A) and lipopolysaccharide (LPS) was significantly enhanced on all days tested following acute administration of malathion. In contrast, subacute treatment with malathion did not affect mitogenic response to Con A or LPS, but led to a significant decrease in thymic cell number.