Eight strains of Clostridium absonum grown in the presence of 4 . 10(-4) M deoxycholate contained both NADP-dependent 7 alpha- and 7 beta-hydroxysteroid dehydrogenase activities. In one strain studied in detail, significant amounts of NADP-dependent 7 alpha- and 7 beta-hydroxysteroid dehydrogenase and NAD-dependent 7 alpha-hydroxysteroid dehydrogenase activities were demonstrated only when cells were grown in the presence of deoxycholate or chenodeoxycholate, both optimal at 4 . 10(-4) M. When the bile salt was deleted from the medium, only a trace of 7 alpha-hydroxysteroid dehydrogenase was present and 7 beta-hydroxysteroid dehydrogenase was absent. Other bile salts including cholate, ursodeoxycholate and keto bile salts were less effective as inducers. Addition of cholate to medium already containing deoxycholate at a suboptimal concentration enhanced the induction, while addition of ursodeoxycholate suppressed the induction. Further enhancement of 7 alpha- and 7 beta-hydroxysteroid dehydrogenase could be obtained by additions of deoxycholate (up to a total of 6 . 10(-4) M) during the growth of the organisms (in log phase). As enzyme enhancement is blocked by addition of rifampicin to the medium, the authors conclude that the enzymes are bile salt-inducible. Growth curve studies revealed an optimal enzyme yield at a harvest time of approx. 6-9 h. We have preliminarily characterized several inducible enzyme components: an NADP-dependent 7 beta-hydroxysteroid dehydrogenase as well as both NAD- and NADP-dependent 7 alpha-hydroxysteroid dehydrogenases.