In the present study, an immunotoxicity test system, containing general toxicological (body weight gain, organ weights), haematological (WBC,RBC, Ht, mean cell volume of the RBCs, cell content of the femoral bone marrow). and immune function (PFC assay, DTH reaction) investigations, was used for detection the effects of a 4 weeks repeated low; dose combined oral exposure of male Wistar rats with propoxur and the heavy metals arsenic or mercury. Two doses of the: compounds were used: a higher one (the lowest dose which resulted in significant change of at least one parameter examined in previous dose-effect experiments), and a lower one (the highest dose which proved to be non-effective). The applied doses were: 8.51 and 0.851 mg kg (1) of propoxur, 13.3 and 3.33 mg kg(-1) of NaAsO2, and 3.20 and 0.40 mg kg(-1) of HgCl2. In the combination treatment. the high dose of propoxur was combined with the low dose of arsenic or mercury. and the high doses of each heavy metals were combined with the low dose of propoxur. The main finding of this study was that some of the combinations significantly altered the relative weight of liver, adrenals and kidneys, related to both the untreated and the high dose internal control. Among the immune functions examined, only the PFC content of the spleen showed a trend of changes in certain combinations versus the corresponding high dose control. According to the present results, combined exposure with propoxur and the heavy metals examined can modify the detection limit of the single compounds and-or may alter their toxic effects. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.