This study was designed to investigate the effects of selenium and aflatoxin on human whole blood cultures (WBC) in relation to induction of sister-chromatid exchange (SCE). The results showed that the frequency of SCEs in peripheral lymphocytes was significantly increased by the direct-acting mutagen AFB(1) (at doses 5 and 10 mu M except for 1 mu M) compared to controls. When sodium selenite (Na2SeO3) was added alone at a molar ratio of 5 x 10(-7) and 1 x 10(-6), cells did not show significant increase in SCE frequency. Whereas, SCE rates induced by the various AFB(1) concentrations could be significantly reduced by the presence of Na2SeO3 in a clear dose-related manner. These results indicated that selenite and AFB(1) mutually antagonized their ability to cause DNA damage leading to the formation of SCEs. However, selenium didn't completely inhibit induction of SCEs by AFB(1) compared to controls. This is first report describing, the protective ability of selenium against AFB(1) genotoxicity on human WBC.