Green tea, one of the most popular beverages consumed in Asian countries, has been reported to possess anticarcinogenic and antimutagenic properties. The aim in this study is to test the radical scavenging effect of catechins and caffeine, which were major components of green tea, and if they really prevent oxygen radical-induced mutagenesis. We used TA102 strain of Salmonella typhimurium which is sensitive to hydroxyl radical in the Ames mutation assay. We found that caffeine did not show any effects on mutagenesis in this system, but catechin significantly reduced mutagenesis or genotoxicity caused by hydroxyl radical. This radical-scavenging action of catechins may indeed contribute to the anticarcinogenic activity of green tea as has been proposed.