IgM rheumatoid factor (RF) was measured in the sera of 48 rheumatoid patients and of 48 age and sex-matched normal controls by the Rose-Waaler and latex agglutination tests, a rate nephelometer, and an enzyme-linked immunosorbent assay (ELISA). Good correlation was obtained between all assays. The rate nephelometer assay was the easiest and quickest to perform and gave results in international units/ml. The Rose-Waaler was the least sensitive assay and the most difficult to perform and interpret. Both the latex agglutination and the ELISA were sensitive, though some overlap of patient and control sera was seen with all the assays. In addition to IgM RF the ELISA was used to measure IgG RF and IgA RF in both rheumatoid and control sera. Although some normal sera had detectable amounts of IgG and IgA RF, the levels of both were significantly raised in the rheumatoid sera. IgG RF levels were lower after pepsin digestion of the sera, suggesting that IgM RF interfered with the assay for IgG RF unless this treatment was included.