When quercitrin was anaerobically incubated with human intestinal becteria, quercetin, 3, 4-dihydroxyphenylacetic acid and 4-hydroxybenzoic acid were found as metabolites. The main metabolite was quercetin. The bacterium transforming quercitrin to quercetin was Fusobacterium K-60. However, Bacteroides, JY-6, which produced α-L-rhamnosidase, did not transform quercitrin to quercetin. Among quercitrin and its metabolites, 3, 4-dihydroxyphenylacetic acid and 4-hydroxylphenylacetic acid had more potent activity than quercitrin on in vitro anti-platelet aggregation activity, and quercetin and 3, 4-dihydroxyphenylacetic acid showed more potent cytotoxicity against tumor cell lines than quercitrin and 4-hydroxylphenylacetic acid.