The interaction of platelets and the vessel wall is of importance in the pathogenesis of intravascular thrombosis and atherosclerosis. We have previously described the presence of an inhibitor of platelet aggregation and a low Km ADPase in aorta intima extracts. The inhibitor was further characterized.Human aorta intimas were homogenized, centrifuged and supernatants used. Platelet aggregation was measured in a aggregometer.ADPase activity was measured by incubating 14C-ADP with intima, separating metabolites by high voltage electrophoresis and quantitated.Aggregation induced by ADP, collagen, Ristocetin and polylysine was inhibited by the intima extract. The inhibition was still evident in aspirin treated platelets. The inhibitor is present in the supernatant of 100 000g centrifuged extracts. This inhibitor appears not to be ADPase or the recently described prostacyclin (PGX) :- it is stable and not destroyed by incubation at 37°C, blocking of prostaglandin synthesis by pre-incubation of the intima with indomethacin has no effect on inhibitor activity.ADPase is inactivated by KCN whereas the inhibitor is not.The inhibitor of platelet aggregation and to a lesser extent ADPase, in the intima may be important regulators of platelet-vessel wall interaction.