FEIBA (factor eight inhibitor by-passing activity) Immuno was used to achieve hemostasis in 46 patients with factor VIII inhibitors with titers greater than 4 Bethesda units, and 3 patients with factor IX inhibitors. One-hundred and sixty-five bleeding episodes were treated with 50-70 U/kg; 102 of these episodes occurred in joints. 20 in mucous membranes, 33 muscle and soft tissue, and 10 were emergency episodes including 3 central nervous system and 4 surgical procedures. Ninety-three percent of the bleeding episodes were controlled, while 7% were not controlled: 36% were controlled by one infusion in 12 hr, another 42% with 1 or more infusions in 36 hrs and an additional 14% were controlled in more than 36 hr. There were no serious side effects, and while the inhibitor titer rose in 10 of the patients, the product continued to be efficacious.