A semi-automated enzyme-linked immunosorbent assay (ELISA) for determination of IgM rheumatoid factor was established. Human gammaglobulin (Cohn fraction II) was used as an antigen on a solid phase of polystyrene microtitre trays. The results were read on a spectrophotometer. ELISA and the sheep cell agglutination test were compared in a study of sera from 400 blood donors, 53 patients with active rheumatoid arthritis and of 200 sera received for routine determination of rheumatoid factor. The results of the two tests correlated well. The ELISA procedure was precise, objective, inexpensive and well suited for quantitative routine determination of IgM rheumatoid factor.