Flavonoid glycosides were metabolized to phenolic acids via aglycones by human intestinal microflora producing ${alpha}$-rhamnosidase, exo-${eta}$-glucosidase, endo- ${eta}$-glucosidase and/or ${eta}$-glucuronidase. Rutin, hesperidin, naringin and poncirin were transformed to their aglycones by the bacteria producing ${alpha}$-rhamnosidase and ${eta}$-glucosidase or endo- ${eta}$-glucosidase, and baicatin, puerarin and daidzin were transformed to their aglycones by the bacteria producing ${eta}$glucuronidase, C-glycosidase and ${eta}$-glycosidase, respectively. Anti-platelet activity and cytotoxicity of the metabolites of flavonoid glycosides by human intestinal bacteria were more effective than those of the parental compounds. 3,4-Dihydroxyphenylacetic acid and 4-hydroxyl-phenylacetic acid were more effective than rutin and quercetin on anti-platelet aggregation activity. 2,4,6-Trihydroxybenzaidehyde, quercetin and ponciretin were more effective than rutin and ponciretin on the cytotoxicity for tumor cell lines. We insist that these flavonoid glycosides should be natural prodrugs.