Dilution with physiologic saline solution and other fluids accelerates the coagulation of properly collected normal and hemophilic blood and plasma in silicone coated vessels, with or without the aid of activating agents such as platelets, thromboplastin, cephalin, glass particles or plasma euglobulin fractions. When normal plasma is diluted under a concentration of about 20 per cent, its rate of clotting is prolonged, principally because of diminution in prothrombin. Regardless of the activating agent used, the rate of coagulation of hemophilic plasma can be made equal to that of normal plasma by appropriate dilution. These findings speak against the existence in hemophilic blood or plasma of a deficiency in any procoagulant factor, and support the concept of the presence in excess of a stabilizing inhibitor which slows the conversion of prothrombin to thrombin by one or both of the following mechanisms: (1) reducing or inactivating the effect of released coagulants (antithromboplastin activity) (2) conjugation with a procoagulant thereby maintaining it in an inactive form (anti Ac-globulin activity).