When ginsenoside R_g3 was anaerobically incubated with human fecal microflora, all specimens metabolized ginsenoside R_g3 to ginsenoside R_h2 and protopanaxadiol. The main metabolite was ginsenoside R_h2. 20(S)-ginsenoside R_g3 was quickly transformed to 20(S)-ginsenoside R_h2 or 20(S)-protopanaxadiol in an amount 19-fold that compared with the transformation of 20(R)-ginsenoside R_g3 to 20(R)-ginsenoside R_h2 or 20(R)-protopanaxadiol. Among the bacteria isolated from human fecal microflora, Bacteroides sp., Eubacterium sp., and Bifidobacterium sp. metabolized ginsenoside R_g3 to protopanaxadiol via ginsenoside R_h2. However, Fusobacterium sp. metabolized ginsenoside R_g3 to ginsenoside R_h2 alone. Among ginsenoside R_g3 and its metabolites, 20(S)-protopanaxadiol and 20(S)-ginsenoside R_h2 exhibited the most potent cytotoxicity against tumor cell lines, 20(S)- and 20(R)-protopanaxadiols potently inhibited the growth of Helicobacter pylori, and 20(S)-ginsenoside R_h2 inhibited H⁺/K⁺ ATPase of rat stomach.