Experiments were conducted to demonstrate that platelet stickiness can be induced by the cooling of a platelet suspension, and that when this system is agitated clumping occurs. The process is reversed by warming. This change is independent of metallic ions and is thus distinct from “viscous metamorphosis.” Platelet clumping in systems at 37 C. is dependent on the presence of divalent metallic ions. The clumping time is also strongly influenced by “contact activation” and by added “activation product.”Other observations throw doubt on the supposed relationship of “viscous (structural) metamorphosis” to thromboplastin formation. The significance of the various observations is discussed, and the phenomena associated with physical and structural changes in the blood platelet