The presence of suppressive antibody activity in sera from patients spontaneously recovered from the Guillain-Barré syndrome was investigated by analyzing the ability of postrecovery serum to inhibit anti-neuroblastoma cell line antibody binding in sera from seven patients in the prerecovery phase or with a chronic form of the disease. All 12 recovered patients analyzed were found to have inhibitory IgG antibodies in their postrecovery sera, of which the F(ab')2 fragments mediated the inhibitory effect. The pattern of inhibition suggests that about half of the patients share cross-reactive idiotypes of high affinity. The efficiency of the inhibition mediated by anti-idiotypic antibodies in spontaneously recovered patients was twice as high as that mediated by anti-idiotypes present in therapeutical preparations of polyclonal immunoglobulins for intravenous use (IVIG). Affinity chromatography of IVIG and serum from a recovered Guillain-Barré syndrome patient on autoantibody-containing F(ab')2 fragments revealed, first, that inhibitory anti-idiotypic antibodies are specifically retained on autoantibodies and, second, that these antibodies constitute less than 1% of the total IgG antibody content.