Nine patients with severe classic hemophilia and inhibitors against factor VIII were treated for 156 bleeding episodes with 503 infusions of Proplex, Konyne, or Auto-Factor IX, three preparations of prothrombin complex concentrates (PCCs). Approximately two thirds of the bleeding episodes were managed successfully. Although the prothrombin time (PT) and partial thromboplastin time (PTT) were shortened after most PCC infusions, there was no evidence of disseminated intravascular coagulation. The degree of shortening of PT or PTT was not related to the particular PCC preparation used, dose, or cessation of hemorrhage. All PCC preparations contained activated clotting factors, as manifested by their ability to shorten the PTT of normal plasma, factor-VIII-deficient plasma, and factor-IX-deficient plasma. Shortening, which was greater with Auto-Factor IX than with the other products, was inhibited partially by a factor IX antibody and blocked completely by prolonged incubation with plasma. Although the nature of the procoagulant material in PCCs is uncertain, these products are of proven benefit to hemophilic patients with high-titer inhibitors. Side effects have been minimal and inhibitor titers have not risen.