In eight patients suffering from severe hemorrhagic disease it was possible to demonstrate the presence of circulating inhibitors, not heparin-like in nature. Anti-PTC was identified in two PTC deficients, anti-AHF in 5 hemophilics and both, primarily anti-AHF, in one idiopathic case. Possible etiologic factors inducing the appearance of such inhibitors included repeated transfusions of whole blood and plasma in all and a previous pregnancy in the idiopathic case. Two patients showed increases of gamma-globulin and presence of inhibitor in 25-33 per cent ammonium sulfate plasma fractions. Cortisone therapy was ineffective in two cases.