In peripheral blood of patients with rheumatoid arthritis the percentages of lymphocytes carrying surface-bound lgG, IgA or IgM did not differ significantly from those in normal controls. However, both the sum of these percentages and the percentage of cells carrying receptors for human C3 were significantly lower than in normal controls. Immunoglobulin and C3 receptor were almost always present on the same cells, indicating that the latter also is a marker for B lymphocytes. In rheumatoid synovial fluids the percentage of lymphocytes carrying immunoglobulin appeared t o be significantly higher than in peripheral blood from the same patients.No similar difference was observed for the percentage of cells carrying C3 receptors, probably due t o impaired detection of this receptor in some of the fluids.Both the infiltration of lymphocytes in rheumatoid synovium and the immunologic phenomena associated with rheumatoid arthritis (RA) suggest that lymphocytes play an important role in this disease. Sevcral lines of evidence indicate that rheumatoid lymphocytes differ from those of normals. They show decreased reactivity in mixed leukocyte cul-