The concentration of beta-2-microglobulin was determined by radioimmunoassay in the synovial fluid of 12 patients affected with rheumatoid arthritis and in 10 with other arthropathies. The white cell count, total protein concentration, total hemolytic activity of complement and acid phosphatase activity were also determined. Only beta-2-microglobulin was found to be significantly higher in rheumatoids compared to non-rheumatoid cases. In contrast, the other parameters showed no significant differences between the 2 groups of patients. These preliminary observations suggest that the measurement of beta-2-microglobulin concentrations in synovial fluid may be of diagnostic value.