Diazinon (DZN), an organophosphate insecticide, has been used in agriculture for several years. It is possible the residue of this compound to be recycled in the biological system. There is no report on DZN immunotoxicity potential. In the present study, we examined the immunotoxic effects of intraperitoneally administered DZN in the C57bl/6 female mice. Diazinon was administered at doses of 25, 2, and 0.2 mg/kg for 28 days (five injections per week). Animals were then sacrificed to observe the cellularity or histopathological changes in thymus, spleen, bone marrow, and peripheral blood. Furthermore, humoral and cellular functional responses such as Hemagglutination titration (HA), IgM-Plaque Forming Colony Assay (PFC), Delayed-Type-Hypersensitivity (DTH) to SRBC, and T cell subtyping (CD4/CD8) were determined. The results showed that DZN at 25 mg/kg not only could produce gross histopathological changes in thymus and spleen but also could suppress both humoral and cellular activity of the immune system. At lower doses (0.2 and 2 mg/kg) there were no observable alteration in cellularity or histology of immune tissues. However, DZN at medium dose (2 mg/kg per day) could inhibit RBC-cholinesterase and showed a mild decrease (P < 0.1) in thymus/body-weight ratio and DTH response. At dose of 0.2 mg/kg no histopathological or functional disturbances were detectable. These results indicate that DZN has immunosuppressive effects in the C57bl/6 mice at doses more than 2 mg/kg. The present results however indicate that under recommended Allowed Daily Intake (ADI) limit (<0.02 mg/kg), no observable immunotoxicity effect is expected.