When ginsenoside R_b1 and R_b2 were anaerobically incubated with human intestinal microflora, these ginsenosides were metabolized to 20-O-β-D-glucopyranosyl-20(S)-protopanaxadiol (compound K) and 20(S)-protopanaxadiol. Several kinds of intestinal bacteria hydrolyzed these ginsenosides. Eubacterium sp., Strep coccus sp. and Bifidobacterium sp., which more potently hydrolyzed gentiobiose than sophorose, metabolized ginsenoside R_b1 to compounds K via ginsenoside R_d rather than gypenoside XVII. However, Fusobacterium K-60, which more potently hydrolyzed sophorose than gentiobiose, metabolized to compound K via gypenoside XVII. Ginsenoside R_b2 was also metabolized to compound K via ginsenoside R_d or compound O by human intestinal microflora. Eubacterium sp., Streptococcus sp. and Bifidobacterium sp. metabolized ginsenoside R_b2 to compound K via ginsenoside R_d rather than compound O. Fusobacterium K-60 metabolized ginsenoside R_b2 to compound K via compound O.