Beta-2-microglobulin was assayed in the plasma and the synovial fluid in 41 subjects with mechanical joint disorders, 27 patients with rheumatoid arthritis and 32 patients with non-rheumatoid arthritis. The plasma beta-2-microglobulin may be raised in all forms of joint disease, especially in the course of rheumatoid arthritis, but its ability to discriminate between rheumatoid arthritis and other forms of inflammatory joint disease is poor. In contrast, for the beta-2-microglobulin level in the synovial fluid, the differences in the means are highly significant between rheumatoid arthritis and non-rheumatoid arthritis. Concentrations of beta-2-microglobulin in the synovial fluid greater than 5.2 micrograms/ml, in this study which excluded patients with renal failure, were 100 per cent specific for rheumatoid arthritis and were found in 52 per cent of cases. However, this result has to be interpreted in the light of the fact that any extra-articular cause for an increased plasma beta-2-microglobulin, particularly renal failure, also causes a rise in the synovial concentration, invalidating the test. For this reason, the authors propose using the value of the beta-2-microglobulin in the synovial fluid minus the plasma beta-2-microglobulin, as a more specific index.