Treatment of individuals with severe hemophilia A by plasma-derived or recombinant factor VIII leads to the production of anti-factor VIII antibodies in approximately 30% of such patients. Because some of these antibodies inactivate factor VIII, they are considered a major factor in preventing optimal therapeutic treatment. Factor VIII is a cofactor that must bind to factors IX and X and phospholipids in order for normal blood coagulation to occur. The inhibition of factor VIII activity is due to binding by anti- factor VIII antibodies in the patient plasma to the same sites required for factors IX and X and phospholipid binding. Previously, inhibitor epitopes were localized to the A2, A3, and C2 domains and to a region of acidic amino acids between the A1 and A2 domains. Inhibitor binding to these domains prevented factor VIII binding to factor IXa (A2, A3), factor Xa (C2), and phospholipids (C2), and binding to the acidic region interfered with factor X binding. Antibody binding to a minor C2 domain epitope slowed activated factor VIII release from von Willebrand factor (vWF) and interfered with factor Xa binding to factor VIII.