In the classical theory of blood coagulation, it was postulated that prothrombin was converted to thrombin through the action of calcium and thromboplastin. It is now established that circulating blood contains no active thromboplastin but that thromboplastin is generated during coagulation. The three major factors required for its production are platelets, factor VIII, and factor IX, the agents lacking in thrombocytopenic purpura, hemophilia, and Christmas disease (hemophilia B), respectively. Biggs and Douglas 1 devised a direct method for measuring the generation of thromboplastin by mixing (1) platelets; (2) diluted plasma adsorbed with aluminum hydroxide which retains its concentration of factor VIII but is devoid of prothrombin and factors VII, IX, and X; and (3) serum which is low in prothrombin, devoid of factor VIII, but rich in factor IX. On recalcification and incubation a thromboplastin activity develops which is measured on normal plasma by a one-stage prothrombin technique. When the