Effects of combined 28 days of oral exposure to the insecticide Permethrin (Pe), alone or in combination with arsenic-III (As) or Hg-II (Hg), were investigated on certain toxicological (body weight, organ weights), haematological (white blood cell (WBC) and red blood cell (RBC) counts, haematocrit (Ht), mean cell volume (MCV). cell content of the femoral bone marrow) and immune function (IgM-PFC, delayed-type hypersensitivity (DTH) reaction) parameters of male Wistar rats. Immunotoxic (H = high) and NOEL (L = low) doses of the three substances were determined in preliminary experiments under identical experimental conditions. In the present study, the immunotoxic dose of Pe (126 mg/kg) was combined with the NOEL dose of As (3.33 mg/kg) or Hg (0.40 mg/kg), and the NOEL dose of Pe (12.6 mg/kg) with the immunotoxic dose of As (13.3 mg/kg) or tig (3.20 mg/kg). A separate group of animals, treated with the appropriate high dose component only, was used as internal control. Significant interactions were observed in the liver weight of the animals treated with Pe(H)-As(L) or As(H)-Pe(L), in the cell content of the femoral bone marrow in case of Pe(H)-As(L) and Pe(H)-Hg(L) combinations, as well as in the number of PFCs formed from 10(6) spleen cells in the Pe(H)-As(L) and in the maximum of DTH reaction in the Hg(H)-Pe(L) combination. The results show that combined exposures by the investigated substances modify the toxic (including immunotoxic) effects of the single compounds. These findings rise the probability that the interactions observed can also be present in human situations altering the health hazard of this three chemicals. (C) 2001 Elsevier Science B.V. All rights reserved.