Cyproterone acetate (CPA), a synthetic steroid used in human therapy, has recently been shown to cause DNA damage in cultured rat hepatocytes and in rat liver. In the present study we have investigat ed whether CPA also induces genotoxic effects in human hepatocytes. Genotoxicity of CPA was deter mined by measuring the capability of the com pound for inducing DNA repair synthesis and for causing the formation of DNA single-strand breaks. Autoradiography and alkaline elution were used to quantitate DNA repair and DNA fragmentation, respectively. Exposure of hepatocytes to CPA for 20 h induced DNA repair synthesis in two hepatocyte preparations derived from males and in four of the five preparations derived from females. In cultures from some donors, induction of repair was detected at 1 μM CPA, the lowest concentration tested. The maximum effect generally occurred at 10-20 μM. Only a very slight increase in the frequency of DNA single-strand breaks was found following exposure of the hepatocytes to 50 μM CPA for 20 h. For com parative purposes, the effects of CPA on DNA repair and DNA fragmentation were also determined in cultured rat hepatocytes. A strong induction of DNA repair synthesis, but only a slight enhancement in DNA fragmentation was observed in CPA- treated hepatocytes derived from female rats. These results indicate that the measurement of repair is a more sensitive indicator for the genotoxicity of CPA than the measurement of DNA fragmentation. No genotoxic effects of CPA were detectable in hepatocyte cultures derived from male rats. The present findings show that CPA is genotoxic in human hepatocytes and that the striking sex difference in the genotoxicity of CPA in rat cells is not observed with human cells.