The long term treatment of hemophilic patients with an inhibitor to Factor VIII has been difficult although some success with immunosuppressive agents has been reported. Eighteen hemophilic patients, mainly from Bonn in Germany, but also from other countries, have completed a high dose Factor VIII treatment in an attempt to reduce their inhibitor titer and induce "immune tolerance" to Factor VIII. Plasma samples from the 18 patients collected before and after infusion of 50 units Factor VIII/kg body weight were sent to five laboratories to evaluate inhibitor titer, Factor VIII recovery and half life. This collaborative study demonstrated a close correlation from one laboratory to another concerning inhibitor titration and Factor VIII recovery. The conclusion of the study is that all patients treated with this protocol showed an undetectable or low inhibitor titer against Factor VIII indicating that they can be efficiently treated with Factor VIII.