Detectability of toxic effects by repeated doses of dimethoate (DM) and methylparathion (MPT) were investigated by geno- and immunotoxicological methods in male Wistar rats following a 28-day oral exposure, In the dose range of 28.2, 14.1, and 7.04, and 7.04 mg/kg/day DM, the two higher doses decreased the body weight gain, The top dose increased the weight of liver, kidneys, and testicles; the white blood cell count; and the cell content of the femoral bone marrow. From immune function parameters measured [IgM-plaque forming cells (PFC) assay, delayed-type hypersensitivity (DTH) reaction] only the maximum of the DTH reaction decreased at the top dose. Of the MPT doses (0.872, 0.436, and 0.218 mg/kg/day) the two higher ones increased the liver weight, and a dose-dependent increase was found in the MCV value. No evaluable changes in the examined immune function parameters were observed. Both substances increased the number of numerical but not the structural chromosome aberretions at lower dose levels (the two larger doses of DM, and all the three doses of MPT) than those ones which caused changes in the examined immune function parameters. According to these results, the genotoxicological approach seems to be more sensitive for detection of repeated-dose oral toxicity of the investigated two organophosphates than the immunotoxicological one. (C) 2000 Academic Press.