The effects of benzyl isothiocyanate (BITC) and benzyl thiocyanate (BTC) on two types of DNA synthesis were examined in hepatocyte primary cultures (HPC). Male F344 rats were fed BITC- or BTC-containing diets at a concentration of 400 p.p.m. Using hepatocytes isolated from these rats, DNA repair was measured by unscheduled DNA synthesis (UDS) for some genotoxic carcinogens, e.g. 2-acetylaminofluorene (AAF), methylazoxymethanol (MAM) acetate, 9,10-dimethyl-1,2-benzanthracene (DMBA) and diethylnitrosamine (DEN), and compared with that in the hepatocytes from rats without BITC or BTC treatment. Replicative DNA synthesis (RDS) was also evaluated in the hepatocytes of rats with or without thiocyanate treatment. Both BITC and BTC reduced UDS elicited by these carcinogens. The level of RDS in the hepatocytes of rats exposed to BITC or BTC was markedly lower than in the cells of rats without BITC or BTC exposure. These results indicate that in vivo exposure to BITC and BTC suppressed carcinogen-induced genotoxicity and cell proliferative activity and suggest that this assay may prove useful in detecting chemopreventive agents for cancer and in investigating the properties of carcinogenesis modifiers.