A panel of 20 human monoclonal antibodies was produced from cells derived from the synovial tissue of two rheumatoid arthritis patients and one polyarticular juvenile rheumatoid arthritis patient. Fourteen IgM monoclonal antibodies were classical rheumatoid factors (RFs) with specificities restricted to IgG and included twelve kappa and two lambda proteins. Two of them were pan-specific for IgG and reacted with all four human subclasses. Nine showed the Ga specificity, i.e. they reacted with IgG1, IgG2 and IgG4 subclass proteins. Two showed the 'new Ga-related' specificity, i.e. they reacted with IgG1, IgG2 and IgG4 subclass proteins, and also with the Ge3m(st) allotype proteins. One RF demonstrated an anti-G3m(u) anti-allotypic specificity. Five IgM(lambda) monoclonal antibodies and one IgM(kappa) monoclonal antibody demonstrated polyreactivity against various antigens including DNA, human thyroglobulin, human serum albumin and tetanus toxoid.