The synthesis of immunoglobulins by cells infiltrating the labial salivary glands has been studied by radioimmunoelectrophoresis in 20 patients with Sjögren's syndrome (SS) and in 14 control patients with related disorders. The patients with SS were producing significantly greater quantities of IgG, IgM, and IgA. Synthesis of IgG and IgM correlated with the degree of lymphoid infiltration but not with serum immunoglobulin concentration. Patients with SS and rheumatoid arthritis (RA) showed greater synthesis of IgG and IgM than those with uncomplicated RA. The only extensive lymphoid infiltration was seen in patients with SS. One patient with SS and primary macroglobulinemia was synthesizing the paraprotein in the lip biopsy as well as in the bone marrow. These results establish the immunologic competence of the infiltrating lymphoid cells and suggest their origin from an extrasalivary source.