Neutralization of factor VIII inhibition by factor VIII peptides. Recombinant A2 (□) or C2 (⋄) or plasma-derived light-chain (○, A3-C1-C2) polypeptides were examined for their ability to neutralize inhibitor patient plasma's factor VIII inhibitor titer. Details are described in Materials and Methods; only the A2 domain neutralized the patient's inhibitor more than 15%. Results are from the sample drawn 2 months after the inhibitor was first detected; similar results were found for the initial sample (not shown). The A2 domain has a molecular weight of 43 kD; light chain is 73 to 80 kD; C2 is 24 kD. Thus, the relative molar amounts of A2 are about half of C2 and twice the light chain at any given concentration.