The protective effects of green tea polyphenols on paraquat-induced genotoxicity in cultured cells were studied. (-)-Epigallocatechin gallate (EGCG), the major constituent of green tea, and (+)-catechin (CT), a minor constituent, equivalently decreased the frequencies of sister-chromatid exchanges (SCE) induced by paraquat (PQ), which is a generator of reactive oxygen species. These polyphenols were effective at concentrations of 1.0 microM and above. A reduction of the effect on the cell cycle rate caused by PQ was found when EGCG and CT were added at concentrations of more than 10.0 microM. These concentrations of EGCG and CT alone had no effect on cell cycle rate, which is used as index of cell proliferation. Decreases in the cell cycle rate were found at 200 microM EGCG and CT in the 24 hr exposure period. The equivalent effectiveness of EGCG and CT suggested the possibility of other mechanisms, apart from acting as reactive oxygen species scavengers, because it has been reported that EGCG is the most potent scavenger among tea catechins. From the present study, it was suggested that green tea and foods containing these polyphenols may be beneficial to human health by protecting against reactive oxygen species-induced genotoxicity.