Feeding of lactose in amounts comparable to the adult human intake in developed countries (6% of diet, and in later studies 10%) had no major effect on cholesterol and bile acid metabolism of germ-free and conventional rats. However, when lactose-containing casein-starch diets were sterilized by autoclaving, changes in intestinal and/or fecal bile acids were found. Both germ-free and conventional rats demonstrated some increase in intestinal beta-muricholic acid concentrations ascribable to the mere presence of lactose in the diet. Autoclaving of the diet produced additional changes, especially in the fecal bile acid pattern of conventional rats. Here the ratio between the beta-muricholic-derived secondary bile acids hyodeoxycholic and omega-muricholic acids changed from the usual 5:3 to approximately 1:10, with omega-muricholic acid becoming the major fecal bile acid. These changes point to a notable effect of lactose-derived products, formed during steam-sterilization, on the microbial modification of intestinal bile acids in the lower gut. Similar changes have been observed after oral administration of aureomycin and other, unrelated antibiotics that inhibit growth of gram positive organisms.