Eight pesticides were tested in a bioassay based on the induction of preneoplastic lesions in the liver. Rats were given diethylnitrosamine intraperitoneally at 200 mg/kg bw and two weeks later were treated with pesticides for six weeks and then killed; all rats had a partial hepatectomy at week 3. Hepatocarcinogenic potential was assessed by comparing the number and area of glutathione s-transferase (placental form) -positive foci in the liver with those of controls given diethylnitrosamine alone. Positive results were seen with Chinomethionat, Phosmet and Propiconazole; the results obtained with Captan and Prochloraz were borderline; Benomyl, Daminozide and Folpet gave negative results. Our findings provide enough experimental evidence to indicate that great care should be exercised in the use of these compounds.