Immunoglobulins, including rheumatoid factors, are produced by the rheumatoid synovial membrane. A significant contribution of the synovial membrane to the total IgG and IgM detected in the synovial fluid has been documented. The present study was designed to examine the contribution of the synovial membrane to the rheumatoid factors detected in the synovial fluid. Analysis of the data demonstrated that the synovial membrane was the source of a significant component of the total synovial fluid IgA rheumatoid factor and IgM rheumatoid factor. While some fluids possessed extremely elevated concentrations of the IgG rheumatoid factor, the data suggested that IgG rheumatoid factor was preferentially reduced, relative to total IgG, by the rheumatoid inflammatory process. These observations suggest a potentially important role for IgG rheumatoid factor in rheumatoid synovitis.The synovial membrane and the synovial fluid are the focal points of the inflammatory process in patients with rheumatoid arthritis. The rheumatoid synovium is capable of locally synthesizing many or all of the immune reactants involved in this condition.