Germfree (GF) mice were orally inoculated with human fecal suspension or various components of human fecal microbiota. Three weeks after the inoculation, cecal bile acid composition of these mice was examined. More than 80% of total bile acids was deconjugated in the cecal contents of ex-GF mice associated with human fecal dilutions of 10(-2) or 10(-6), or anaerobic growth from a dilution of 10(-6). In these ex-GF mice, deoxycholic acid accounted for about 20% of total bile acids. In the cecal contents of ex-GF mice associated only with clostridia, unconjugated bile acids made up less than 40% of total bile acids, about half of those in other ex-GF groups. However, the percentage of deoxycholic acid in these mice was the same as that in the other groups. These results indicate that dominant anaerobic bacterial combination is efficient for deconjugation of primary bile acids, and that clostridia in the human feces may play an important role in 7alpha-dehydroxylation of unconjugated primary bile acids in the intestine.