When ginsenoside Rb1 and Rb2 were anaerobically incubated with human intestinal microflora, these ginsenosides were metabolized to 20-O-beta-D-glucopyranosyl-20(S)-protopanaxadiol (compound K) and 20(S)-protopanaxadiol. Several kinds of intestinal bacteria hydrolyzed these ginsenosides. Eubacterium sp., Streptococcus sp. and Bifidobacterium sp., which more potently hydrolyzed gentiobiose than sophorose, metabolized ginsenoside Rb1 to compound K via ginsenoside Rd rather than gypenoside XVII. However, Fusobacterium K-60, which more potently hydrolyzed sophorose than gentiobiose, metabolized to compound K via gypenoside XVII. Ginsenoside Rb2 was also metabolized to compound K via ginsenoside Rd or compound O by human intestinal microflora. Eubacterium sp., Streptococcus sp. and Bifidobacterium sp. metabolized ginsenoside Rb2 to compound K via ginsenoside Rd rather than compound O. Fusobacterium K-60 metabolized ginsenoside Rb2 to compound K via compound O.