This research project seeks a better understanding of the mechanism by which human antibodies inactivate factor VIII procoagulant activity. Procoagulant and immunologic methods will be used to investigate three aspects of this immune interaction: the antigenic properties of the immune complexes fomed, factors which influence the rate of the interaction and the manner in which this interaction may interfere with the coagulant function of factor VIII. Solid phase immunoadsorbent reagents to which factor VIII is nonconaletly linked will be utilized to study the effect on binding of anti-factor VIII antibody has on the structure of factor VIII complex. Radiolabeled human antibody will also be used as a probe for the mechanism of proteolytic activation of factor VIII.