An enzyme-linked immuno-sorbent assay (ELISA) for quantitation of rheumatoid factors (RF) of IgM, IgA and IgG isotypes has been established. A complex of human serum albumin (HSA) and rabbit IgG anti-HSA antibodies is used as antigen for RF. The binding of RF is detected by stepwise additions of biotinylated monoclonal antibodies specific for human IgM, IgG or IgA, alkaline phosphatase-conjugated streptavidin, and substrate. The assay is simple and applicable to routine testing of large numbers of sera. It discriminates between false and true IgG-RF by papain digestion of sera that turn out positive by the screening for IgG-RF. Of 241 randomly selected patients with rheumatoid arthritis (RA) as well as other rheumatoid and infectious diseases, 110 were Waaler-Rose-positive while 127 were IgM-RF-positive in ELISA. The correlation between the Waaler-Rose test and IgM-RF ELISA was highly significant (r = 0.82). By testing 65 of these sera (all IgM-RF positive), 25 (39%) were also true IgG-RF positive (42 (64%) in the screening). When 40 Waaler-Rose-positive RA patients were tested, 20 and 21 were also positive for IgG- and IgA-RF, respectively. Eight IgM-, one IgA- and no IgG-RF positive tests were recovered when 48 Waaler-Rose negative RA patients were studied.