Factor VIII corrects both the clotting defect in hemophilia A and an abnormality of platelet aggregation in von Willebrand's disease. These two activities of factor VIII (antihemophilic factor and von Willebrand factor) are both detected in the void volume when human plasma or cryoprecipitate is chromatographed on Bio-Gel 5M under conditions of isotonic salt concentration. In contrast, antihemophilic factor procoagulant activity is detected with proteins of lower molecular weight when the chromatography is performed with a buffer containing 0.8M NaCl. In this way, the two activities of factor VIII can be dissociated. It remains to be determined whether these components are separate molecules associated as a complex of high molecular weight in plasma or whether they are subunits of a complex macromolecule.