Previous studies have shown that highly concentrated blood platelets behave in anticoagulant fashion. In the present investigation fractionation of human blood platelets yielded two anticoagulant fractions: one lipid, the other evidently protein in nature. The lipid anticoagulant inhibited both thromboplastin formation and the action of formed thromboplastin in the thromboplastin generation test. It was selective neutralized by Ac-globulin concentrate. The protein anticoagulant interfered only with thromboplastin formation and was neutralized by the factors in serum which adsorb on barium sulfate. It is suggested that the platelet anticoagulants may be partially responsible for the hemorrhagic symptoms of thrombocythemia. Submitted on November 29, 1956