Clostridium absonum was shown to metabolize primary bile acids to give rise to both 7-oxo bile acids and 7 beta-hydroxy (urso) bile acids. At relatively low redox potential (Eh) values, high yields of urso bile acids were achieved (60-75%). If, however, the Eh value of the culture was allowed to rise above approximately -100 mv, the 7-oxo bile acid would tend to predominate (more than 75%) and the "death phase" was accelerated. Growth of C. absonum in sterile graduated cylinders instead of in conventional Erlenmeyer flasks was effective in delaying the rise in Eh value with time (which appears largely due to diffusion of atmospheric oxygen into the medium) and in preserving a higher viable count of organisms. It is proposed that the formation of excess amounts of 7-oxo bile acid is a manifestation of oxygen toxicity and that it could be mediated by an increasing intracellular NADP:NADPH ratio. Additionally, the reaction: primary bile acid in equilibrium oxo bile acid in equilibrium urso bile acid was shown to be partially reversible. When the organisms were grown with [24-(14)C]chenodeoxycholic, -cholic, or -7-keto-lithocholic acid, this reaction could be clearly demonstrated. The addition of an equimolar concentration of deoxycholic acid (which itself is not metabolized) effectively enhanced the rate of bioconversion of cholate and 7-keto-lithocholic, but not chenodeoxycholate (whose rate of bioconversion was the fastest of the three). When the organisms were grown with urso bile acids (ursocholic or ursodeoxycholic) or with 7-keto-deoxycholic acid, very little metabolism occurred unless deoxycholic acid was added which induced formation of primary and keto bile acids. In all cases, formation of oxo bile acid from primary or urso bile acid occurred as the Eh value of the medium rose with time and could thus be delayed by the use of a cylinder instead of a flask for growing the culture. These results were rationalized by demonstrating that induction of 7 alpha- and 7 beta-hydroxysteroid dehydrogenase is strongly mediated by chenodeoxycholic and deoxycholic acids, weakly mediated by cholic and 7-keto-lithocholic acids, and ineffective with 7-keto-deoxycholic, ursocholic, and ursodeoxycholic acids.