Seven patients with hemophilia A and B and 3 patients with acquired hemophilia were treated on 13 and 4 occasions, respectively, with protein A immunoadsorption to reduce anti-factor VIII or IX antibodies. Usually the patients were treated on 2 consecutive days. On each treatment day an average of 3 (range: 1.02-5.83) plasma volumes were processed in the congenital patients and 1.5 (range: 1.02-2.90) in those with acquired hemophilia. Plasma levels of IgG decreased from 18.9 +/- 1.9 to 3.1 +/- 1.2 g/l in the congenital group, and from 11.5 +/- 2.3 to 2.3 +/- 0.6 g/l in the acquired group. In the congenital hemophiliacs a corresponding reduction in inhibitor level of 70-95% was regularly seen; in 1 exceptional patient the inhibitor was reduced from 4,350 to 12 Bethesda Units/ml (BU/ml) during 5 days of treatment. In the congenital hemophiliacs immunoadsorption was followed by factor infusion to peak levels between 8 and 215 IU/dl. In the patients with acquired hemophilia a satisfactory reduction in inhibitor levels was obtained in 2 of the 4 treatments, which were followed by DDAVP or factor infusion. Some recommendations for the use of protein A immunoadsorption in the treatment of hemophilic patients will be given.