The hybridoma, 62H3, which secretes a monoclonal IgG2b with anti-HLA-DR specificity, was expanded in pristane-primed BALB/c mice and the antibody was isolated from the ascitic fluid by affinity chromatography on Protein A-Sepharose. The purified IgG2b antibody was tested by an enzyme immunoassay for antibody activity against a panel of 40 self and non-self antigens. It was found to react strongly with beta-galactosidase, actin, glutamate dehydrogenase, rabbit and human IgG and di- and trinitrophenyl groups; and moderately with tubulin, insulin and phosphorylcholine; but it did not react with various other self and non-self antigens, such as DNA, albumin, keyhole limpet hemocyanin, hen lysozyme and horseradish peroxidase. Fab and Fc fragments were prepared from this IgG2b by papain proteolysis. The Fab fragment possessed the same spectrum of polyreactivities as the native IgG2b, whereas no activity was detected with the Fc fraction. In order to investigate the properties of the antigen binding site, the actin, TNP and rabbit IgG antibody activities were studied in more detail by enzyme immunoassay, Western blot and immunocytochemistry. The monomolecular nature of this multireactivity was confirmed by immunoabsorption analysis. Furthermore, 62H3 monoclonality was also verified by comparative isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis with other monospecific antibodies. The dissociation constants (Kd) of antigen-antibody equilibria in solution were measured. The Kd for actin was 1.11 +/- 0.24 x 10(-5) M and the Kd for TNP-BSA was 8.7 +/- 0.51 x 10(-7) M. No interaction with rabbit IgG could be detected in solution. These findings raise the question of the possible implication in autoimmune pathology or in normal physiology of IgG class polyspecific antibodies with solid-phase restricted cross-reactive rheumatoid factor activity.