Abstract: Objective To evaluate immunotoxic effects of beta -cypermethrin on male mice.Methods Delayed type hyper-sensitivity (DTH) was investigated with footpad swelling assay.Antibody production(SRBC-PFC) was assayed with quantitative hemolysis spectrophotometry(QHS) and serum haemolysin titers were performed with direct agglutination(hemaggluti-nation).Moreover,The levels of lipid peroxidation in thymus and spleen tissue homogenates were measured with thiobarbi-turic acid(TBA) method.Results Antibody secreted by plaque forming cells decreased,the immune response determined by antisheep erythrocyte was hindered and the level of lipid peroxidation in spleen increased.However,no effects on DTH and lipid peroxidation in thymus were observed.Conclusion Humoral immune suppression was induced by beta-Cypermethrin in male mice,which is in relation to the toxic mechanism of lipid peroxidation