Gradient centrifugation was quite useful in obtaining "mononuclear" cell fractions from synovial fluids derived from patients with rheumatoid arthritis. The ratio of lymphocytes to monocytoid cells was similar to that seen in peripheral blood. Prior treatment of the fluid with hyaluronidase was found to be necessary to obtain a satisfactory yield of the cells. Sheep erythrocyte rosette-forming lymphocytes (T-cells) were studied in 18 paired samples of synovial fluid and blood in RA and 15 control blood samples from normal persons were also examined. In the paired synovial fluid and blood samples from the RA patients, the mean percentages of the T-cells were 56.6 and 47.1 respectively and the difference was statistically significant (0.01