The metabolic capacity to activate the indirectly acting promutagens aflatoxin B1, cyclophosphamide, benzo[a]-pyrene, 7,12-dimethylbenz[a]anthracene and dimethylnitrosamine into DNA-reactive metabolites was investigated in three immortalized rat hepatocyte cell lines (NRL cl-B, NRL cl-C and ARL) by analysing chromosome aberrations and sister chromatid exchange (SCE). In all three cell lines a significant clastogenic and SCE inducing response was observed after exposure to each test compound. Furthermore, activities of the two enzymes aryl hydrocarbon hydroxylase and aldrin-epoxidase, which play major roles in the cytochrome P450-dependent metabolism, could be determined in all cell lines. In contrast to the hepatocyte lines in V79 Chinese hamster cells, which were used as a reference cell line without any cytochrome P450 metabolizing capacity, no arylhydrocarbon hydroxylase or aldrinepoxidase activities were detected. A cytogenetic response to the test compounds was only observed in the presence of the exogenous activating system S9 mix. Due to the wide, efficient and stable spectrum of their metabolizing capacities, the tested rat hepatocyte lines offer promising perspectives as alternative assay systems for the detection of indirectly acting mutagens.