Human intestinal flora from normal subjects was incubated with chenodeoxycholic acid (CDCA), ursodeoxycholic acid (UDCA) and 7-ketolithocholic acid (7-keto-LCA) for various periods of time and the metabolites thus formed were quantitatively determined by gas chromatography (GC) and identified by gas chromatography-mass spectrometry (GC-MS). Results showed that: a) the 7 alpha-dehydroxylation of CDCA produced more lithocholic acid more rapidly than the 7 beta-dehydroxylation of UDCA; b) the oxidation of CDCA produced more 7-keto-LCA more rapidly than the oxidation of UCDA; and c) the transformation of 7-keto-LCA produced CDCA and UDCA peaks after 8 h of incubation and a lithocholic peak at the end of 24 h of incubation after rising from almost 0% at 4 h. The presence of 3 alpha-hydroxy-delta 6-5 beta-cholenic acid, a hypothetical unsaturated intermediate in the 7-dehydroxylation of CDCA and UDCA, could not be detected by GC-MS.