In vitro autoantibody production induced by lipopolysaccharide (LPS) was studied using peripheral blood mononuclear (PBM) cell suspensions from patients with rheumatoid arthritis (RA) and healthy subjects. PBMs from both groups could be induced by LPS to secrete IgM and IgA rheumatoid factors (RF), antinuclear and anti-beta-2-microglobulin autoantibodies. Spontaneous production of IgM-RF was considerably higher in RA than in controls. The rate of IgM-RF and IgA-RF secretion detected by ELISA increased with the dose of LPS in cultures of both groups. In RA, differences were found between the kinetics of IgM- and IgA-RFs secretion. LPS augmented the relative avidity of IgM-RF produced by PBMs from RA patients and this value was significantly higher than that of healthy persons. In some cases RFs cross-reacting with nuclear antigens and beta-2-microglobulin were detected.