L ymphocyte heterogeneity was studied in peripheral blood and salivary gland lesions in 24 patients with Sjögren's syndrome. Peripheral blood B cells, measured by immunofluorescence with specific antiserum to immunoglobulins or by rosette assay with complementcoated erythrocytes, were increased in most patients. Peripheral blood T cells, measured by immunofluorescence with rabbit antiserum to human thymocytes or by rosette assay with sheep erythrocytes, were reduced in eight patients. Three had associated rheumatoid arthritis, two had a generalized lymphoproliferative disorder, and one each had scleroderma, systemic lupus erythematosus, and neuropathy.The salivary gland lymphocytic infiltrates present in labial biopsy specimens were compared in 10 patients using an indirect immunofluorescent method with anti-human T cell serum and a quantitative focus-scoring method. In general, there was a correlation between the number of T cells and the extent of the infiltrate. Striking accumulations of T cells were present in some patients, but clusters of presumed B cells were also seen. These results indicate an increase in peripheral blood B cells in most patients, a decrease in T cells in some, and a mixed T and B cell infiltrate in the salivary gland lesions.