Concentration of four immunoglobulins and orosomucoid, transferrin, ceruloplasmin, and α-2 macroglobulin were estimated in sera and synovial fluids in patients with rheumatoid arthritis and in degenerative joint disease. An increase in serum immunoglobulins in rheumatoid arthritis was found in 15 per cent for immunoglobulin G (IgG), 21 percent for IgA, and 6 per cent for IgM. Low IgA level was observed in 19 per cent of estimations. IgD was either normal or low. Although the level of immunoglobulins in rheumatoid synovial fluids was generally lower than in the corresponding sera, it exceeded that in the serum in 8 to 16 per cent. No relationships between the concentration of immunoglobulins to the age, sex, duration of the disease, sedimentation rate, titer of latex fixation, Lansbury index, or extent of the damage in two simultaneously aspirated joints were found. Comparison of the serum/synovial fluid ratios of various proteins has shown that in rheumatoid synovial fluids all proteins tested are present in higher concentration than in the synovial fluids in degenerative joint diseases. The ratios for IgG and to the lesser extent for IgM exceeded those theoretically calculated for their molecular weights. These observations imply both excessive exudation of plasma proteins to the synovial fluids and production in situ of immunoglobulins.