Recent studies have shown that various dietary regimens and many pharmacologic agents influence serum-lipid concentrations in man. Ingestion of diets high in unsaturated fat1or administration of neomycin2or large doses of nicotinic acid3usually cause a decrease in the levels of cholesterol and other lipids in serum. Little is known of the mechanisms responsible for the effects of these dissimilar agents. One mechanism which could lead to a decrease in serum-cholesterol concentration would be an increase in fecal excretion of sterols and bile acids, the latter being the principal endproducts of cholesterol metabolism. This possibility is the subject of the present report. Human subjects were maintained on controlled diets high in saturated or unsaturated fat and were given neomycin or nicotinic acid during part of each experimental period. Fecal excretion of sterols and bile acids were determined with methods developed in our laboratory. Methods Methods for