Triazine herbicide has been reported to directly suppress the immune response. In the present study, the effect of simazine on the immune response was investigated. Splenic lymphocytes were treated withmitogen (lipopolysaccaride, concanavalin A) in the presence of simazine. When simazine(300 mg/kg, 600 mg/kg) was administrated every day for 3 weeks or 4 weeks, respectively, the number of plaque forming cells (PFC) was decreased. Antibody production of IgM and IgG class was significantly decreased in splenic cells from simazine-treated animals. In addition, when animals were exposed to simazine, the susceptibility of virus infection as well as the growth of tumor cells was increased. These data suggest that simazine affected the immune function and humoral immunity impaired by simazine treatment contributed to pathological process.