Rutin which is metabolized by intestinal lacteria, not by mammlian digestive enzymes, is known to be promutagenic in Salmonella/mammalian microsome assay. The mutagenic metabolite of rutin by intestinal bacteria was quercetin. However, on the present study, the mutagenic quercetin was metabolized to an unknown compound, 2, 4, 6-trihydroxybenzoic acid and 3, 4-dihydroxybenzoic acid by intestinal bacteria and the mutagenecity of quercetin was also decreased according to producing the metabolites. These metabolites of quercetin seemed to be related to the pharmcological activity. The metabolites, 2, 4, 6-trihydroxybenzoic acid and 3, 4-dihydroxybenzoic acid, have a good antico agulating activity. In addition, some flavonoids, hesperidin, naringin and their aglycones have antimutagenic activity more than the mutagens. These results suggest that dietary flavonoids were metabolized to some compounds with the pharmacological action more than the chronic genetic toxicants by intestinal microflora.