Muscle biopsies from 21 dermatomyositis (DM) and 7 polymyositis (PM) patients were studied by conventional histoenzymatic reactions and immunoreacted with antibodies against T cells and subsets, B cells, macrophages, activated T cells, proliferating cells, transferrin and IL-2 receptors, and natural killer cells. The expression of both class I and II molecules from the major histocompatibility complex (MHC) was also tested. As control groups we used muscle biopsies from normal healthy people, from chronic alcoholics and from a cohort of HIV-1 infected patients. In DM cases, severe muscle fiber necrosis, predominant perivascular infiltrates, fibrosis and perifascicular atrophy were the rule whereas in PM cases, endomysial infiltrates and the existence of partially invaded non-necrotic cells were more frequent. Perivascular B cells were found only in some DM cases. Transferrin and IL-2 receptors, proliferating cells and NK cells were detected in some cases from both diseases. MHC class I molecules were detected mainly in perifascicular fibres in DM while in PM the stronger expression was demonstrated in non-necrotic partially invaded cells, suggesting for the latter a MHC-restricted T-cell cytotoxicity. MHC Class II molecules expression in endothelial cells was detected in a variable fashion in both diseases, probably reflecting different stages of activation of such cells.