The capability of two rat hepatoma cell lines, H4IIEC3/G- and 2sFou, to detect genotoxicity of xenobiotics, was evaluated in a micronucleus assay. In this system, the cells act as the activation source as well as the target cells for the DNA damage. The study was performed using 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene and cyclophosphamide, as pro-mutagens and mitomycin C as a direct acting mutagen. In both cell lines a significant micronucleus induction was observed after exposure to each test compound, starting from 25 nM for 7,12-dimethylbenz[a]anthracene, 8 microM for benzo[a]pyrene, 0.5 mM for cyclophosphamide and 0.4 microM for mitomycin C. A period of 24 h treatment and 48 h growth was sufficient for induction and expression of micronuclei. The two hepatoma lines behave in a similar way with regard to the pro-mutagen activation. The results obtained in this study with these differentiated hepatoma lines demonstrate that they are metabolically competent to activate the promutagens 7,12-dimethylbenz[a]anthracene, benzo[a]pyrene and cyclophosphamide into their biologically active metabolites as measured by micronucleus induction in our experiments. However, the variable dose responses to 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene in some of the repeated experiments, suggest unstable activity of enzymes involved in polycyclic aromatic hydrocarbons metabolism in these cell lines.