Abstract: Objective To observe the effect of diazinon on cellular inmunity and humoral inmunity of Balb/c mice.Methods Diazinon was gavaged to mice for 4 weeks at dosages of 1.6,3.2,6.3,12.5 and 25 mg/kg The inm une function was evaluated with Tlymphocyte proliferation,N K cell activity assay,IgM-laque foun ing cell assay (PFC),blood serumerythrocytolysin assay and delayed-type-hypersensitivity (DTH)to SRBC.Results 6.3 mg/kg diazinon decreased the number of IgM-laque founing cell,blood serum erythrocytolysin level and DTH to SRBC.12.5,25 mg/kg dizainon could decrease the relative weight of thymus at about 20%,the number of IgM-laque founing cell at about 39% and 52%,the blood serum erythrocytolysin level at about 15% and 20%,the DTH to SRBC at about 33% and 35%,the ability of Tlymphocyte proliferation at about 27% and 35%,the activity of NK cell at a bout 24% and 28%,respectively.Conclusion Diazinon could inhibit humoral inmunity,cellular inmunity and non-specfic inm unity of Balb/c mice Humoral inmunity is more sensitive to diazinon toxicity.