Dogs, exposed to X-irradiation, developed a hemorrhagic state which was characterized by a prolonged bleeding time, a prolonged clotting time, thrombocytopenia, and the presence of an anticoagulant in the blood indistinguishable from heparin. Toluidine blue or protamine sulfate could temporarily control the bleeding tendency. On empiric grounds, these substances were used in the treatment of thrombocytopenic purpura in man and found to temporarily control the formation of petechiae although the bleeding times were not shortened. A method for the estimation of heparin tolerance in these patients was worked out, using "back titration" principles where it was detd. how much protamine sulfate was necessary to restore the clotting time to normal in blood to which known amts. of heparin had been added. The thrombocytopenic patient''s blood had a reduced tolerance for added heparin.