Plasma samples obtained from a patient 6 wk, 6 months, and 4 yr after recovery from anti-VIII:C (anti-hemophilic factor, where VIII:C = factor VIII procoagulant activity) autoimmune disease were found to contain antibodies that inhibited anti-VIII:C activity in the patient's prerecovery plasma and in the plasma of two other patients with anti-VIII:C autoantibodies. F(ab')2 fragments from postrecovery IgG suppressed anti-VIII:C activity in F(ab')2 fragments from prerecovery IgG within a narrow range of molar ratios. Anti-VIII:C activity in F(ab')2 autoantibodies was also inhibited by F(ab')2 fragments from polyspecific therapeutic immunoglobulins prepared from a large pool of normal donors (IVIg). IgG from prerecovery plasma bound to F(ab')2 from postrecovery IgG and to F(ab')2 from IVIg, as assessed by ELISA. Affinity chromatography experiments demonstrated that F(ab')2 from postrecovery IgG preferentially bound anti-VIII:C antibodies among F(ab')2 fragments from prerecovery plasma containing anti-VIII:C autoantibodies. F(ab')2 from prerecovery plasma bound in higher amounts to postrecovery F(ab')2 than to IVIg. Insolubilized F(ab')2 fragments from postrecovery plasma also bound F(ab')2 fragments prepared from the plasma of another patient with anti-VIII:C autoimmune disease, although in lesser amounts than the patient's own prerecovery anti-VIII:C F(ab')2 antibodies. These observations suggest that human anti-VIII:C autoantibodies share idiotypic determinants and that spontaneous recovery from anti-VIII:C autoimmune disease occurs through idiotypic suppression of autoantibodies. In patients who recover from autoimmune disease and in patients in whom autoantibodies have been suppressed by infusions of IVIg, antiidiotypic antibodies, possibly by providing internal images of the antigen, may have shifted the immune system toward the steady-state equilibrium that prevents autoimmunity in normal individuals.