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SUMMARY: Certain anaerobic bacteria from human faeces were found to be able to hydrogenate cholesteroli n vitro; some also decreased to a marked degree the total amount of sterols in the incubation mixture. Other organisms did not hydrogenate or decrease the amount of sterols under the conditions chosen; among these were; Clostridium welchii, C. sporogenes, Bacterium bifidum, various streptococci and micrococci, Escherichia coli, Aerobacter aerogenes. A ' germ-free ' filtrate from human faeces was inactive. In a series of incubated samples, where the Tschugaeff reaction was applied to the non-saponifiable matter, an orange colour developed, suggestive of the presence of lathosterol.That hydrogenation of cholesterol can be carried out with human faeces or colon contents in vitro was demonstrated by Dam (1934b). At the same time it was shown that faeces heated at 100' for 20 min. failed to hydrogenate cholesterol. Certain bacteria in pure culture, such as Escherichia coZi and a type B enterococcus were inactive. Later, Rosenfeld, Fukushima, Hellman & Gallagher (1954 a, b ) succeeded in converting cholesterol into coprosterol with human faeces in vitro. Rosenheim & Webster (1943) found that the bacteriostatic agent succinylsulphathiazole inhibited coprosterol formation in vivo in rats, but they also observed inhibition by the amoebicidal agent p-carbaminophenylarsonic acid which did not suppress the coliform flora. They therefore questioned the participation of bacteria in the process. The present work shows that cholesterol can be converted into coprosterol by some anaerobic bacteria from human faeces, whereas other such bacteria are inactive in this respect. METHODSComposition of media. Suspensions of brain were used as cholesterolcontaining substrate. Rosenheim & Webster (1941) were of the opinion that cerebrosides such as those in brain are necessary for the hydrogenation of cholesterol in the intestine. However one of us (Dam, 1934a) found that, in man, faecal cholesterol was hydrogenated to more than S O . / , without ingestion of cerebrosides. Further, Rosenfeld et aZ. (1954a, b ) used substrates without added cerebrosides in their in vitro experiments. The reason why we used brain suspensions as substrate is that such substrates are an easy means of providing a colloidal suspension of cholesterol which at the same time contains nutrients for bacteria. Desiccated hog or ox brain (2.5 or 5 g. portions) were stirred with tap water to make 5 % (w/v) suspensions. In some cases, 1 yo (w/v) peptone and 0.1 yo (w/v) glucose were also added. Undissolved