We analyzed the rheumatoid factors (RF) produced by Epstein-Barr virustransformed monoclonal B cells established from four patients with rheumatoid arthritis (RA), three individuals with a history of Mycobacterium tuberculosis (TB) and four normal controls (NI). Fifty-eight R F were analyzed for specific activity (international units-RF/pg) for the Fc part of IgG and their interaction with tetanus toxoid (TT) and DNA (polyspecificity). Furthermore, we sequenced the V-D-J heavy chain region of 16 (9TB-/7RA-) RF. Significant differences were observed between the NI-RF and the TB-and RA-RE While the R F repertoire of normal individuals comprised of low-avidity R F of which the majority (15/17) were polyspecific, more than half of the TB-and RA-RF were monoreactive. Furthermore, the monospecific TB-and RA-RF were of significantly higher avidity than the NI-RF (RA > TB >> NI). With respect to polyspecificity, the R F in the three groups were comparable: the interaction with DNA, TT as well as with Fc was inhibited either by an increase of the ionic strength to 0.3-0.5 M NaCl or by addition of the polyanion dextran sulfate, indicating that the antibodies interacted with similar anionic epitopes shared by the three antigens. Analysis of the V-D-J heavy chain regions showed significant differences between the respective RF. The salt-sensitive binding was highly correlated with the presence of arginine in the complementarity-determining region 3 (CDR3). Furthermore, whereas the polyspecific R F consisted predominantly of germ-line encoded antibodies, the genes of the monospecific RARB-RF were somatically mutated (RA > TB). It is therefore likely that maturation of RF can be initiated by chronic infections and that monospecific, somatically mutated R F are not a unique characteristic of autoimmune diseases.