The immunotoxic effect of paraquat (PQ), a herbicide that has been used widely in agriculture was investigated using Balb/c mice. Paraquat was administered at doses of 1, 0.1, and 0.01mg/kg for 21days. Body weight, organ weight, cellularity of spleen, delayed type of hypersensitivity (DTH) response, plaque-forming cell (PFC) assay, hemagglutination titer (HA), quantitative hemolysis of SRBC (QHS) assay, spleen cell subtypes, cytokine production and lymphocyte proliferation assay were studied in various groups of animals. Results showed that high dose of PQ (1mg/kg) could suppress both cellular and humoral activity of the immune system. PQ at medium dose (0.1mg/kg) did not show any changes in organ weight, body weight and spleen cellularity but significantly decreased the proliferation response to PHA and the production of IFNγ. PQ at low dose (0.01mg/kg) did not produce any significant changes in humoral or cellular responses of the immune system. In conclusion, paraquat at high dose has an inhibitory effect on the cell-mediated and humoral immunity. It seems that PQ has no adverse effects on mice immune system at low doses of 0.01mg/kg, which is two times the PQ allowed daily intake (ADI) limit.