Chlorpyrifos, o,o diethyl-o-(3,5,6-trichloro-2-pyridyl) phosphorothioate, is a broad spectrum organophosphate insecticide. The use of chlorpyrifos has been increased more and more as pesticide. But the effects of chlorpyrifos on the immune alterations has not been yet observed. Therefore, we investigated the effects of chlorpyrifos on the immune alterations in CICA male mice. Chlorpyrifos was administered to mice by a single intraperitoneal injection for the purpose of observing acute effects. On the one hand to get the information on immunopathologic alterations we observed hematological values, counted total circulating leukocytes and assessed the ratio of lymphocytes and neutrophils from the peripheral blood, measured the ratio of organ/body weight and counted splenic cellularity in CBA male mice which treated chlorpyrifos intraperitoneally. But we could not find any significant immunopathologic alterations statistically by a single intraperitoneal injection. Also, the exposure of chlorpyrifos caused no significant change in the number of PFC/10$^6$ spleen cells at any three given doses. On the other hand a singte intraperitoneal injection of chlorpyrifos decreased the lymphocyte proliferation response slightly to ConA or LPS stimulation at a dose of 6 mg/kg b.w. Administrations of chlorpyrifos reduced mixed leukocyte response(MLR). MLR was decreased moderately at doses of 3mg/kg b.w. and 6mg/kg b.w. Therefore, all these findings suggest that chlorpyrifos may alter the immune functions acutely. especially by the changes of T lymphocyte activity.