We prepared antiidiotypic (anti-Id) antibody to 2 polyclonal IgM rheumatoid factors (IgM-RF) and 2 polyclonal "hidden" IgM-RF. The anti-Id antibodies were isolated by chromatography on Sepharose 4B, to which was bound rabbit anti-human IgG Fc fragments. F(ab')2 fragments from the anti-Id antibodies were generated by pepsin digestion and isolated by gel filtration. The anti-Id antibodies directed against RF from 4 patients with juvenile rheumatoid arthritis (JRA) were tested by an inhibition hemolytic assay for cross-reactivity with IgM-RF from 4 adult patients with rheumatoid arthritis, 6 patients with JRA, and 13 JRA patients with hidden RF. The 4 anti-Id antibodies had variable cross-reactivity with the isolated adult RA RF, JRA RF, and JRA hidden RF. Similar results were obtained by a direct-binding enzyme-linked immunosorbent assay for the anti-Id antibodies. The broad pattern of cross-reactivity was apparently unrelated to a particular amino acid sequence, but was associated with the antigen-binding site of IgM-RF. These results suggest the possibility that the anti-Id antibodies prepared against isolated RF obtained from JRA patients bear the "internal image" of antigen; that is, the Fc region of human IgG. These anti-Id antibodies may be generated in JRA patients and may possess specific immunomodulatory properties.