Lymphocyte subsets were determined in the peripheral blood from twenty-three patients with primary Sjögren's syndrome (SS) and sixteen patients with clinically active rheumatoid arthritis (RA) by two-color flowcytometry using various monoclonal antibodies. In both diseases, T-cells (CD3+), suppressor/cytotoxic cells (CD8+) and their cytotoxic subset (CD8+CD11-) were decreased, as compared with thirty-one healthy controls. B-cells (CD 21+ and CD 3-DR+) and activated T-cells (CD 3+DR+) were increased in SS patients. Helper T-cells (CD 4+Leu8-), suppressor-inducer T-cells (CD4+Leu8+), suppressor T-cells (CD8+Leu 15+) and three natural killer (NK) cell subsets determined by both CD16 and Leu7 antibodies did not differ between controls and SS or RA, although Leu7+NK cells were significantly increased in SS patients. In addition, we found that the treatment with low-dose prednisolone decreased B-cells and suppressor-inducer T cells, and increased suppressor T-cells, cytotoxic T-cells and Leu7+NK cells. The results indicate similar changes in the proportion of lymphocyte subsets and suggest immunologically activated and deficient conditions in both diseases. Immunomodulating effects of the treatment with low-dose prednisolone on some of the lymphocyte subsets in patients with these diseases were also supported by the study.