Renal interstitial cells play an important role in renal function and renal diseases. We describe the morphology of renal interstitial cells in the healthy kidney. We distinguish within the renal interstitium (1) renal fibroblasts and (2) cells of the immune system. Fibroblasts are in the majority and constitute the scaffold of the kidney; they are interconnected by junctions, and are attached to tubules and vessels. Although the phenotype of fibroblasts shows some variation depending on their location in the kidney and on their functional stage, their recognition as fibroblasts is possible on account of structural features. Among the cell types of the second group, antigen-presenting dendritic cells are the most abundant in in the peritubular interstitial spaces of healthy kidneys. Their incidence is highest in the inner stripe of the outer medulla. They share some morphological features with fibroblasts but lack others--junctional complexes, morphologically defined connections with tubules and vessels, and the prominent layer of actin filaments under the plasma membrane--that are characteristic for fibroblasts. Dendritic cells in healthy kidneys are morphologically different from macrophages, which are characterized by abundant primary and secondary lysosomes. In healthy kidneys macrophages are restricted to the connective tissue of the renal capsule and the pelvic wall, and to the periarterial connective tissue. Lymphocytes are rare in healthy kidneys. The distinction of cell types by morphology is supported by differences of membrane proteins. Among all interstitial cells in the renal cortex, fibroblasts alone exhibit ecto-5'-nucleotidase. Dendritic cells constitutively have a high abundance of MHC class II protein. Both proteins are mutually exclusive. Rat macrophages display the membrane antigen ED 2 and lymphocytes exhibit specific surface antigens, depending on their type and functional stage, e.g., CD4 or CD8.