Washed platelets activated by alpha-thrombin, gamma-thrombin, thrombocytin or the ionophore A23187 (ref. 3) lose their disk shape, produce pseudopodia and become cohesive. This cohesiveness is accompanied by the expression of an endogeneous haemagglutinin which, although apparently bound to the platelet membrane, is dependent on cell secretion. The interaction of this agglutinin with appropriate receptors on other platelets is believed to be responsible for aggregation. We report here that platelets can be prepared which lack agglutinin activity but have receptor function, that afibrinogenaemic platelets lack receptor activity, and that fibrinogen is the receptor for the agglutinin secreted by activated platelets.