Nine fecal samples from four healthy subjects were examined for their ability to transform chenodeoxy-cholic acid (CDCA) and ursodeoxycholic acid (UDCA) in in vitro anaerobic broth cultures. Seven samples converted CDCA and UDCA into each other (more than 50% of CDCA was converted into UDCA while 10% or less of UDCA was converted into CDCA), and produced 7-keto-lithocholic acid and lithocholic acid equally from both acids. No alteration of the 7 beta-hydroxy group of UDCA was demonstrated by two fecal samples that failed to perform mutual 7-epimerization, suggesting the conversion of UDCA into lithocholic acid via CDCA. The 3 alpha-hydroxy groups of these substrate and metabolite bile acids were invariably partially epimerized to 3 beta-hydroxy groups by all the fecal samples. Evidence is presented for the prevalence of these 7- and 3-epimerizing organisms among the human intestinal flora.