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Prostacyclin (prostaglandin I2, PGI2), produced by the blood vessel wall is the most potent known inhibitor of platelet aggregation induced by such stimuli as ADP and thrombin. It binds to a specific platelet receptor and activates adenylate cyclase, raising the cyclic AMP level in platelets. This property can be important because platelets participate in several significant interactions. For example, the interaction with fibrinogen or fibrin contributes to the formation of the haemostatic plug. Intact plasma fibrinogen is required for the aggregation of platelets induced by ADP, and endogenous platelet fibrinogen influences thrombin-induced aggregation. We have therefore studied the effect of prostacyclin on the interaction of fibrinogen with human platelets. We now report that prostacyclin inhibits the mobilisation of specific binding sites ('receptors') for fibrinogen on human platelets and that this effect parallels the inhibition of ADP- or thrombin-induced aggregation. The inhibitory effect of prostacyclin may limit the extent of platelet-fibrinogen interaction in vivo and in extracorporeal circulation.