The immunotoxic effect of a 28 days oral exposure by 55.4, 22.2, and 11.1 mg/kg cypermethrin (CY) was investigated in 4 weeks old male Wistar rats. The applied test system involved the determination of general toxicological parameters (body weight gain, organ weight of thymus, heart, lung, liver, spleen, kidneys, adrenals and the popliteal lymph node), haematological parameters (white blood cell count, red blood cell count, haematocrit, mean cell volume of red blood cells, cellularity of the femoral bone marrow), as well as immune function assays (splenic plaque forming cell assay, delayed type hypersensitivity reaction). The highest dose resulted in a significant increase of the relative liver weight, and all three doses resulted in (although inconsistent) changes in the haematocrit and MCV values. The maximum of DTH reaction decreased at all three doses. On combination of the highest CY dose with non-effective doses of lead or cadmium the immunotoxic effects of the former were modified. When immunotoxic doses of Cd or Pb were combined with the lowest CY dose, further interactions were observed on the examined parameters. The alterations of the immunotoxic effects of CY by simultaneous exposure with Cd or Pb, as described here, can lead to unexpected health consequences and/or can lead to false positive or negative results in human epidemiological studies. (C) 1999 International Society for Immunopharmacology. Published by Elsevier Science Ltd. All rights reserved.