Immunoprecipitation of inhibitor patient antibodies bound to factor VIII peptides. Increasing volumes of inhibitor patient plasma were added to and incubated with 0.75 nmol/L ¹²⁵I-factor VIII fragments in solution phase and labeled fragments bound to antibodies were precipitated by binding to protein G-Sepharose beads (see Materials and Methods). (A) and (B) are results from the inhibitor patient anti–factor VIII plasmas from initial (0) and 2-month samples, respectively, after the inhibitor was first detected (see Fig 1). Polypeptides were plasma-derived A1 (□), A2 (⋄) and light chain (○), and recombinant C2 (▵).