Sterol-dependence of gastric protective activity of unsaturated phospholipids. The major aim of this study was to investigate the gastric protective effect of unsaturated phospholipids and to determine the ability of neutral lipids to enhance this activity. We found that although a liposomal suspension of unsaturated phosphatidylcholine (PC) administered intragastrically failed to protect rats from acid-induced gastric ulcer formation, addition of cholesterol to unsaturated PC induced a dose-dependent protective response with the maximally effective dose, reducing lesion score greater than 70%. This effect also was seen with the plant sterol, beta-sitosterol (reducing lesion score by 81.6 +/- 36%) but was blocked if cholesterol was esterified to fatty acids of varying chain length. Addition of sterols to liposomes of saturated dipalmitoylphosphatidylcholine, in contrast, attenuated the gastric protective action of the saturated PC. It appears that the protective mechanism elicited by sterols and unsaturated PC is not mediated by alterations in gastric emptying rate or prostaglandin biosynthesis, although maintenance of surface hydrophobicity may be involved. These results suggest that the sterol may promote the packing of adjacent unsaturated phospholipid molecules of either the cell membrane or a putative extracellular hydrophobic lining of the gastric epithelium to provide the mucosa with protection against luminal acid. 