The main bile acids in human bile are cholic, chenodeoxycholic, and deoxycholic acids, l which are present as conjugates with taurine or glycine. Cholic and chenodeoxycholic acids are the “primary” bile acids formed from cholesterol in the liver, whereas deoxycholic acid is a product of microbial dehydroxylation of cholic acid in the intestine during the enterohepatic circulation of bile (1). Intestinal microorganisms also cause a dehydroxylation of chenodeoxycholic acid yielding lithocholic acid (2). However, lithocholic acid is reabsorbed only to a limited extent and is therefore practically absent from bile. In addition to these dehydroxylations, the bile acids are further modified by intestinal microorganisms and are excreted in feces as a complex mixture of predominantly free bile acids (l-4). Several of the bile acids present in human feces have been identified. Carey and Watson (5) isolated deoxycholic acid and Heftmann et al.(6) described the isolation of lithocholic, isolithocholic, and l2-ketolithocholic acids. Recently, Rosenfeld and Hellman (7) reported that deoxycholic and lithocholic acids were the main bile acids in human feces. Gas ehromatographic analyses of methylated extracts of human feces revealed, however, the presence of several additional compounds with retention times similar to those of bile acid methyl esters, and a more extensive study of the nature and origin of the fecal bile acids therefore seemed of importance, especially in view of the investigations currently conducted in this laboratory on the influence of diet and hormones on the turnover and excretion of bile acids in man. Sccording to current concepts, cholic and chenodeoxycholic acids are not metabolically interconvertible in man (1, 2), and it should therefore be possible to establish the origin of the different fecal bile acids from either cholic or chenodeoxycholic acid by administering at the same time these two acids labeled with different isotopes. This would also enable the simultaneous determination of the turnover of these two acids in the same