The cellular composition of the salivary gland infiltrates in primary Sjögren's syndrome and the presence of lymphokines were evaluated using the Avidin-Biotin-Peroxidase technique in 10 biopsies from patients with Sjögren's syndrome. The 10 specimens were stained with monoclonal antibodies (MoAb) against T-lymphocytes (Leu-4), Monocytes/Macrophages (M phi) (Leu-M3 and OKM1), Natural Killer cells (NK) (Leu-11 and Leu-7), class II MHC antigens (anti HLA-DR and OKIa1, anti Leu-10), Interleukin-2 (anti IL-2), IL-2 Receptor (anti IL-2 receptor) and Interferon gamma (IFN-gamma) (supernatant fluid from hybridoma cell line producing MoAb to IFN-gamma). The predominant cell was the T-helper/inducer lymphocyte. T-helper/T-suppressor ratio ranged from 1/1 to 10/1. Only few M phi were observed while NK cells were rare. More than 50% (50-100%) of the infiltrating cells were HLA-DR positive while epithelial cells were positive from 40-90% in all but two cases in which the positivity was 10%. The infiltrating cells were also positive for Leu-10 antigen (40-95%) in contrast to the epithelial cells. Both infiltrating and epithelial cells were reacting with anti IL-2 MoAb in high percentages (50-100%), while none of the infiltrating cells expressed IL-2 receptor. Finally, 40-100% of the infiltrating cells were IFN-gamma positive. Our data indicate that the salivary gland lesion in Sjögren's syndrome, is the result of an immune process in which T-lymphocytes predominate producing lymphokines, while the epithelial cells by expressing class II MHC molecules, occupy an important part of the picture.