A patient with atypical rheumatoid arthritis (RA) and local synovial synthesis of oligoclonal IgG in an arthritic knee joint is described. Measles virus-specific antibodies isolated from the synovial fluid (SF) were carried by oligoclonal IgG proteins but constituted only a fraction of the total oligoclonal IgG of the SF. Smooth muscle antibodies were markedly increased in the SF compared with the serum and were associated with an electrophoretically restricted fraction of IgG. The results indicate that a local synovial synthesis of measles virus-specific antibodies and of smooth muscle antibodies occurred within the affected joint in our patient.