Abstract The platelet-clumping substance isolated from rabbit plasma by the authors aggregated washed rabbit platelets, while ADP did not. The presence of AMP or adenosine did not inhibit the appearance of platelet aggregation induced by the clumping substance. Pretreatment of phosphoenol-pyruvate and pyruvate kinase on unwashed platelets did not affect the platelet aggregation by the clumping substance. It clumped human platelets and also shrimp thrombocytes. Electron microscopic findings showed that platelets to which the clumping substance was added aggregated in their original form as falt disc shapes with few pseudopods. While fusion of membrane was present, microtubules were observed in its regular form. These results showed the existence of a platelet-clumping substance in blood that is different from ADP, collagen, thrombin, adrenaline etc. and may play an important role in hemostasis and thrombogenesis.