The effect of cholesterol-enrichment in platelet membranes on U46619 binding to the specific receptor and phospholipase A2and C activities was studied using cholesterol-loaded human platelets prepared by in vitro incubation with cholesterol-rich liposomes. The cholesterol-enriched platelets, having a higher cholesterol/phospholipid molar ratio, were hyperaggregable to collagen, arachidonic acid, the thromboxane mimetic U46619 or thrombin. The number of binding sites for U46619, but not the affinity, was significantly increased. Arachidonic acid liberation from membrane phospholipids in response to collagen, thrombin or A23187 was also markedly increased. Furthermore, GTPγS-induced stimulation of the platelet membranes isolated from the cholesterol-enriched platelets, caused significantly increased arachidonic acid liberation but not increased diacyglycerol formation, as compared with the membranes from normal platelets.These results suggest that a certain physical change in cholesterol-loaded membranes brings about a hyperresponsiveness of the endoperoxide receptor and a hyperreactivity of phospholipase A2, probably through enhancement of the coupling efficiency of the corresponding GTP-binding protein to the enzyme, thereby resulting in the increased aggregability to collagen or arachidonic acid. The increased sensitivity to thrombin may be due to a factor such as an increase in thrombin binding, since phospholipase C activity was not enhanced in response to stimuli without intervention of the receptor.