IgM and IgA rheumatoid factor (RF) synthesis by synovial membrane mononuclear cells was measured in 14 patients with rheumatoid arthritis (RA). The results were compared with blood mononuclear cell cultures and correlated with the intensity of lymphocyte infiltration of the synovium. IgM RF was produced by all synovial cultures compared with 56% of blood cultures; IgA RF was produced by 86% of synovial cultures and by 21% of blood cultures. A correlation was observed between synovial IgM RF synthesis, but not IgA RF synthesis, and the intensity of T cell and B cell infiltration of the synovial membrane.