Of the many theoretical causes of rheumatoid arthritis(RA), the most widely held theory is the autoimmune mechanism. In order to clarify the clinical significance of the immunological tests in RA, we studied immunoglobulin and complement levels in sera and synovial fluids of 118 RA patients and the following results were obtained. 1) The levels of immunoglobulins were elevated in both serum and synovial fluid and this was more prominent in the seropositive cases than the seronegative ones. 2) The levels of C3 component were decreased in both serum and synovial fluid, while those of C4 were decreased only in synovial fluid. Serum C3 and C4 component levels were more decreased in the seropositive cases than the seronegative ones. 3) The immunoglobulin levels in serum (IgG, IgM and IgA) and synovial fluid (IgG and IgA) and the levels of C3, C4 component in serum were well correlated with the clinical forms of rheumatoid arthritis. 4) The IgA level in serum and IgM level in synovial fluid were more increased in the exacerbated cases than the chronic ones. 5) Serum IgG level was decreased after steroid medication over one month.