The course and treatment of a life-threatening hemorrhagic episode in a patient with hemophilia A whose plasma contained a high concentration of an inhibitor of factor VIII activity is presented. The inhibitor of factor VIII was localized to the most anodal fractions of immunoglobulin G on electrophoresis, and was thus presumed to be an antibody directed against factor VIII. No therapeutic benefit occurred with infusions of massive amounts of fresh blood and factor VIII concentrates, or with a brief course of immunosuppressive therapy. Administration of standard and activated prothrombin complex concentrates resulted in reduction of the partial thromboplastin time to almost normal values and control of hemorrhage. Eight months later, another hemorrhagic episode occurred. Although a higher titer of inhibitor of factor VIII activity was still present in the patient's plasma, a beneficial therapeutic response was again achieved with standard prothrombin complex infusions.