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The 7 alpha-dehydroxylation of chenodeoxycholic acid (CDCA) and cholic acid (CA) by a Eubacterium lentum-like intestinal anaerobe was specifically enhanced by the bacteroides present in mixed cultures and also by the addition to the growth medium of cell extracts from the bacteroides. The 7 alpha-dehydroxylating organism also possessed 7 alpha-hydroxysteroid dehydrogenase activity, and, in collaboration with a 7 beta-dehydrogenating organism, converted ursodeoxycholic acid (UDCA) into CDCA. Large quantities of lithocholic acids were produced from UDCA as well as CDCA in in vitro cocultures of these three kinds of microorganisms.