The purpose of this work is to review the arguments which support the role of mixed micelle formation in the biliary secretion of lipids. These arguments are derived from in vitro physicochemical studies and from results obtained in vivo during biliary drainage in animals and in man. They show that, for the essential, mixed micelle formation between lecithins, cholesterol and bile salts can explain the biliary lipid secretion. The amount of lipids transported into the bile depends on the intrahepatic metabolism of cholesterol and lecithins. Different bile salts have opposite effects on the saturation of bile with cholesterol. During chronic administration of bile salts, the differences may be explained by specific actions on cholesterol metabolism and particularly on cholesterol absorption. On the contrary, during acute injection of bile salts, in most animal species, those bile salts which have the greatest ability of dissolving cholesterol in vitro (dihydroxy being more efficient than trihydroxy) are those which determine the greatest biliary secretion of cholesterol.