In the present study, the frequency of sister chromatid exchanges (SCEs) was assayed to evaluate the genotoxic effects of mercury nitrate (Hg2+), methylmercury chloride (CH3HgCl and phenylmercury acetate (PMA) on human lymphocytes. The free radical scavengers, catalase (CA) and superoxide dismutase (SOD) were tested for their antigenotoxic effects toward PMA. PMA (1-30 microM) increased SCE frequency in a concentration-dependent manner. However, CH3HgCl significantly increased SCE frequency only at a concentration of 20 microM, and all concentrations treated with Hg2+ did not induce a positive effect. On the other hand, we first reported that 30 microM Hg2+, 20 microM CH3HgCl and (3-30 microM) PMA significantly increased the frequency of endoreduplicated mitosis. PMA was about 3- or 5-fold more effective in inducing endoreduplication than CH3HgCl or Hg2+ at equivalent toxic concentrations, respectively. However, neither CA nor SOD in concentrations of 75 and 150 microg/ml showed antagonistic action on the genotoxic effects of PMA. The results suggest that the mechanism of PMA-induced genotoxicity is not mediated by superoxide anion nor H2O2. It is concluded that PMA, which was more effective in inducing the elevation of both SCEs and endoreduplication, may be especially hazardous of the three mercury compounds tested.