A quantitative "sandwich" enzyme-immunoassay for IgM rheumatoid factor (RF) using human immunoglobulin G as substrate covalently bound to small plastic discs is described. The findings indicate that it is a simple, sensitive, reproducible, and specific method for measuring RF activity. Test results are reported in IU/ml on the basis of the World Health Organization reference RF serum, permitting standardized reporting of this determination between laboratories using this method. Interassay and interlaboratory variability of results are less than 7%. The enzyme-linked immunosorbent assay (ELISA) results correlated well with those found by latex and bentonite flocculation titrations. A number of ELISA RF-positive specimens that were latex-negative and/or Waaler-Rose-negative were confirmed positive by inhibition of the assay with soluble aggregated human IgG, but not with nonaggregated human IgG. Comparison was made between ELISA RF results when using goat or human IgG as substrate. Human IgG was usually more reactive with RFs than was goat IgG.