The organophosphorous insecticide acephate was tested for its ability to induce in vitro cytogenetic effect in human peripheral lymphocytes by using the chromosomal aberrations (CAs), sister chromatid exchange (SCE) and micronuclei (MN) assay. The level of nuclear DNA damage of acephate was evaluated by using the comet assay. Concentrations of 12.5, 25, 50, 100 and 200 mu g mL(-1) of acephate were used. All concentrations of acephate induced significant increase in the frequency of CAs and in the formation of MN dose dependently (r = 0.92 at 24 h, r = 0.95 at 48 h for CAs, r = 0.87 for MN). A significant increase was observed in induction of SCE at 50, 100 and 200 mu g mL(-1) concentrations during 24 h treatment and at all concentrations (except 12.5 mu g mL(-1)) during 48 h treatment period in a dose-dependent manner (r = 0.84 at 24 h, r = 0.88 at 48 h). Acephate did not affect the replicative index and cytokinesis-block proliferation index (CBPI). However, it significantly decreased the mitotic index at all three highest concentrations (50, 100, 200 mu g mL(-1)) for 24 h treatment and at all concentrations (except 12.5 mu g mL(-1)) for 48 h treatment, dose-dependently (r = 0.94 at 24 h, r = 0.92 at 48 h). A significant increase in mean comet tail length was observed at 100 and 200 mu g mL(-1) concentrations compared with negative control in a concentration-dependent manner (r = 0.94). The mean comet tail intensity was significantly increased at only 200 mu g mL(-1) concentration. The present results indicate that acephate is a clastogenic, cytotoxic agent and it causes DNA damage at high concentrations in human lymphocytes in culture.