We designed a double-blind trial to study the effect of an "activated" prothrombin-complex concentrate (FEIBA) on joint and muscle bleeding in hemophiliacs with antibodies to factor VIII. Fifteen patients received either FEIBA or the control preparation (a nonactivated prothrombin-complex concentrate) for a total of 150 bleeding episodes (four mucocutaneous bleeding, 117 joint bleeding, and 29 muscle bleeding). In 64 per cent of the episodes, FEIBA was judged by the physician to be effective; the control preparation was perceived as effective in 52 per cent of the episodes in which it was used. Pairwise comparison of FEIBA and the control preparation for bleeding in the same joint or muscle showed a significantly better result with FEIBA (P = 0.0085). Joint mobility after the use of FEIBA was significantly improved (P = 0.006). There was a high incidence of hepatitis (three of the 15 patients) and of transient disturbances of liver function (nine of 15) during the 15-month observation period. (N Engl J Med. 1981; 305:717–21.)