Animal milk was tested directly for antiulcer activity in a rat model system. The severe mucosal necrosis and intragastric bleeding induced by the luminal administration of supraphysiologic 0.6 N HCl was significantly reduced (50%-70%) if both suckling rats and adults were pretreated per os with raw rat or bovine milk. Pasteurized/homogenized bovine milk also gave protection, although to a lesser extent (30%-50% reduction in bleeding). Treatment of milk with cholestyramine removed 50%-80% of the surface-active phospholipids from milk and also eliminated milk's protective property, an effect that was reversed upon addition of phospholipids to the extracted milk. The antiulcer action of milk in rats could be related to its concentration of dipalmitoyl lecithin. These findings suggest that milk has a potent antiulcer activity that may be attributable to its phospholipid constituents.