Antihemophilic factor (Factor VIII) is an agent in normal plasma that corrects the coagulative defect of classic hemophillia. The plasma of hemophiliacs contains normal amounts of a variant of antihemophilic factor deficient in clot-promoting properties. In contrast, von Willebrand's disease is usually associated with a true deficiency of this protein. In this disorder, the platelets are poorly aggregated by ristocetin, a defect ascribed to deficiency of antihemophilic factor. Structural studies of antihemophilic factor suggest that it is composed of two dissociable subcomponents, one of high molecular weight that contains the bulk of protein and sustains ristocetin-induced platelet aggregation, and another of lower molecular weight with procoagulant activity. Both subcomponents have been identified in hemophilic plasma, as if the smaller subcomponent were qualitatively abnormal. Carriers of hemophilia can often be detected because their plasmas contain a disproportionately high concentration of antihemophilic factor, measured immunologically, compared with the titer of procoagulant antihemophilic factor.