Addition of 0.1 per cent MER-13 to an atherogenic, high-cholesterol diet largely prevented the development of hypercholesteremia and aortic and coronary artery atheromatosis in cockerels. The hypocholesteremic effect was not obtained when MER-13 was administered following the removal of cholesterol from the diet. It is concluded that MER-13 inhibits absorption of cholesterol from the gastrointestinal tract.