Enzyme-linked immunosorbent assays (ELISA) were developed for the measurement of rheumatoid factors (RF) of IgM, IgA and IgG isotypes. Rabbit IgG was used as substrate and the results expressed as units by referral to standards. Rabbit antibody could not be used for detection of IgA or IgG RF without either pepsin-digesting the antibody or inactivating the IgM RF activity of the test sample. When a double antibody system of mouse monoclonal anti-human Fab gamma and goat (Fab')2 anti-mouse IgG was used for the IgG RF assay, pepsin treatment of serum samples was unnecessary. Raised serum IgA RF may be a reliable early marker of erosive rheumatoid arthritis (RA).