In this study, the effects of chlorpyrifos and dichlorvos that are organic phosphorous insecticides were investigated on humoral immune system during 28 days (chlorphyrifos at 10, 20, 40 and 80 mg/kg/bw/day; dichlorvos at 30, 60 mg/kg/bw/day). For this aim, a total of 80, Swiss Albino race, male, three-months-old mice were used. Mice were immunized with S. thyphi intraperitonally at the 1^st and 15^th day of experiment. Following that, on 13^th (primary period) and 28^th (secondary period) days, blood samples were taken and antibody titers were evaluated. As a result, statistically significant (p<0.05) suppression in humoral immunity was determined in groups including 80 mg/kg/bw/day with drinking water at first period, and including 40 mg/kg/bw/day and 80 mg/kg/bw/day chlorpyrifos and 60 mg/kg/ca/day dichlorvos at second period.