Characterization and distribution of interstitial cell types in the renal cortex of rats. A precise classification of the cellular components of the cortical peritubular interstitium is missing in spite of its physiological and pathophysiological importance. By combining electronmicroscopy and immunohistochemistry we attempted to better identify and localize the different interstitial cell types in rat kidney, with special attention to the cortex. Antibodies against common leukocyte antigen (OX1), a macrophage antigen (ED 2), MHC class II antigen (OX6) and ecto-5′-nucleotidase were applied to cryostat sections. The two latter antibodies were used additionally in electron microscopy. Four groups of cell types were identified that displayed distinct antigenic patterns, distributions and morphological features. They were identified as fibroblasts, dendritic cells, macrophages and lymphocytes. Fibroblasts and dendritic cells constituted the most abundant cell types in the cortex and outer medulla, and they were rather uniformly distributed in these zones throughout the peritubular interstitium, where macrophages were very scarce. The latter were abundant in the connective tissue of large vessels and of the pelvic wall. The incidence of lymphocytes was generally low. These data may be helpful on the one hand for the further phenotypic characterization of renal interstitial cells and on the other hand for the evaluation of the roles of those cells in pathological processes.