Ruminococcus sp. PO1-3, an intestinal bacterium isolated from human feces, metabolized glycyrrhizin (GL) to glycyrrhetic acid (GA) and GA to 3-oxo-glycyrrhetic acid (3-oxo-GA) and possessed GL β-D-glucuronidase and 3β-hydroxysteroid dehydrogenase (3β-HSD) involved in the metabolism of GL. This bacterial growth was enhanced by GL at a concentration of 0.4 mM and was suppressed by GA at concentration of 1.0 mM. henodeoxycholic acid, deoxycholic acid and lithocholic acid among the bile acids added to this bacterium suppressed the growth ad GL β-D-glucuronidase activity and 3β-HSD activity incident to it at a concentration of 1.0 mM, while cholic acid, hyodeoxycholic acid and glycine and taurin conjugates of cholic acid, chenodeoxycholic acid, deoxycholic acid and lithocholic acid had almost no effect on this bacterium at a concentration of 2.0 to 1.0 mM. However, these enzyme activities of this sonicated bacteria were inhibited by all of these bile acids. Althrough each bile acid and GL added to bacteria at the same time supressed the growth and te amount of metabolite GA by all bile acids used except cholic acid, taurocholic acid and taurodeoxycholic acid with GL, a combination of each bile acid and GA eased the growth inhibition caused by GA at a concentration of 2.0 mM and enhanced the amount of metabolite 3-oxo-GA by the glycine conjugate of bile acids with GA. GL or GA added after 6 h culture with each of these bile acids and bacteria was metabolized to a relatively large amount of GA by chenodeoxycholic acid and lithocholic acid and their glycine and taurine conjugates, glycocholic acid and taurodeoxycholic acid, or had almost no effect on the amount of metabolite 3-oxo-GA, respectively. These results showed that although GL added after the exposure to bile acid and GA and bile acid added at the same time as bacteria had different bile acid action, these conditions enhanced the amount of metabolite GA from GL and metabolite 3-oxo-GA from GA.