The peripheral blood from 15 patients with primary Sjögren's syndrome and from 15 matched controls was examined for the presence of activated T and B lymphocytes, by using monoclonal antibodies directed to interleukin-2 (IL-2) and transferrin receptors, and to HLA-DR determinants. The number of circulating positive-T cells was significantly greater in the patients than in the controls, irrespective of disease activity. There were more of the CD8 cells than of the CD4 cells that expressed IL-2 receptors. There was a small but significant increase in activated B cells in the patients, since this population is virtually absent from the normal blood.