Intravenous immunoglobulin (IV Ig) is useful in most patients with spontaneous factor VIII:C (FVIII:C) inhibitors, but some complete failures also are observed. Among patients responding to this therapy, decreases in FVIII:C autoantibody titer occurs within 24-48 hours and may lead to suppression of inhibitor activity. The prolonged response observed in some cases suggest an effect on autoantibody synthesis. The immediate decrease in FVIII:C inhibitory activity after IV Ig infusion indicates a direct interaction between IV Ig and the autoantibody. This effect is reproducible in vitro by mixing the patient's plasma and Ig at an appropriate molar ratio, which differs in each patient. Similarly, incubation of Ig with F(ab)'2 fragments of a patient's IgG and Ig reproduces inhibition, and this result indicates that the interaction is mediated by antigen binding sites (epitopes) on the immunoglobulins. The suggestion is that an idiotype- anti-idiotype mechanism must be at work. The origin of such anti-idiotypes in Ig prepared from pooled plasma of several thousand blood donors is unclear. The F(ab)'2 fragments were prepared from individual blood donors and tested in similar experiments with F(ab)'2 fragments of three distinct spontaneous FVIII:C inhibitors. The plasma level of anti-idiotypic antibodies reacting with FVIII:C inhibitors varied according to age and gender.