A method to standardize the quantitation of IgM-rheumatoid factor (RF) by enzymeimmunoassay (EIA) is presented. Serially diluted sera from rheumatoid arthritis patients were added to immobilized human IgG, and bound IgM-RF was detected by addition of horseradish peroxidase labeled anti-human IgM (HRPOaM). The concentration of IgM-RF which produced half of the maximal absorbance at 492 nm given by a saturating concentration of IgM-RF in the EIA plate, was defined as 1 U/ml. The IgM-RF values of test samples were measured as the dilution of the sample which provided half-maximal absorbance. The IgM-RF values determined by this method coincided with those determined by referring to a standard curve made from a serum containing known amounts of IgM-RF. Differences in IgM-RF values, which were caused by varying preparations of horseradish peroxidase anti-IgM (HRPOaM) were corrected for the binding capacity of each preparation to various concentrations of human IgM adherent to the plate. The IgM-RF values determined by this method correlated well with the RF values determined by latex photometric immunoassay (r = 0.956, p less than 0.001). IgM-RF values determined by EIA were converted into WHO-units by an empirical formula described. The data observed suggest that the method here reported can standardize IgM-RF values obtained by EIA.