Ruminococcus sp. PO1-3 from human intestinal flora reduced dehydrocholic acid to 3β-hydroxy-7,12-dioxo-5β-cholanic acid by means of the enzyme 3β-hydroxysteroid dehydrogenase (Akao, T., Akao, T., Hattori, M., Namba, T. and Kobashi, K. (1986) J. Biochem. (Tokyo) 99, 1425–1431). This bacterium and its crude extract gave rise to another product, showing a lower R F value on TLC, from dehydrocholic acid. The product was identified as 3β,7β-dihydroxy-12-oxo-5β-cholanic acid. The crude extract reduced 7-ketolithocholic acid and its methyl ester, but not 6-ketolithocholic acid and 12-ketochenodeoxycholic acid, in the presence of NADPH, and oxidized ursodeoxycholic acid and β-muricholic acid, but not cholic acid, chenodeoxycholic acid, deoxycholic acid and hydrocholic acid, in the presence of NADP +. Therefore, besides 3β-hydroxysteroid dehydrogenase, 7β-hydroxysteroid dehydrogenase was shown to be present in this bacterium. The two dehydrogenases were clearly separated from each other by butyl-Toyopearl 650 M column chromatography. From dehydrocholic acid, 7β-hydroxy-3,12-dioxo-5β-cholanic acid was produced by 7β-hydroxysteroid dehydrogenase and 3β,7β-dihydroxy-12-oxo-5β-cholanic acid was produced by combination of two enzymes, 7β- and 3β-hydroxysteroid dehydrogenase.