The aim of this study was to investigate the genotoxic effects of potassium bromate, which is used as a bleaching agent in flour, on human peripheral blood lymphocytes in vitro by sister chromatid exchange (SCE), chromosomal aberrations (CA) and micronucleus (MN) tests, and also to determine whether it has any genotoxic potential for humans. Cells were treated with 400, 450, 500, 550 microg/ml concentrations of potassium bromate for 24 and 48 h. The SCE frequencies showed an increase after both treatment periods, however, the differences between the treated cells and the control groups were found to be statistically significant only for the 48-h treatment. In addition, potassium bromate statistically significantly induced CA after the 24-h and 48-h treatment periods. Strikingly, potassium bromate induced CA as much as the positive control, mitomycin-C (MMC). Furthermore, potassium bromate decreased both the cell proliferation index (PI) and the mitotic index (MI). Although micronucleus formation was induced by potassium bromate during the 24-h treatment period in a dose-dependent manner, only the doses 500 and 550 microg/ml yielded statistically significant results. In contrast, MN formation was significantly induced at all doses during the 48-h treatment period. These in vitro results provide important evidence about genotoxicity of potassium bromate on a human cell culture system.