Study of the platelets from the viewpoint of the blood coagulation mechanisms seems likely to be helpful for increasing our knowledge of thrombosis, and bleeding tendencies in thrombasthenias and thrombopathias. A logical approach to their study is the differentiation of their properties and where possible to ascribe those properties to individual components. Platelet factors 1, 3 and 4 are considered from that viewpoint. The first accelerates the formation of thrombin in a manner analogous to serum Ac-globulin. Factor 3 is required for the development of threone activity, and factor 4 has antiheparin properties.