Submicroscopic findings of T lymphocytic subpopulations of normal human peripheral blood defined by OKT monoclonal antibodies were studied using ultrastructural immunocytochemistry (immunogold staining). The results show that OKT4-and OKT8-positive lymphocytic subpopulations have a distinct morphological pattern, although some variations in the ultrastructural details of cells in each subset are evident. The most representative features of OKT8-positive cells are regular or slightly indented nucleus with condensed chromatin, abundant cytoplasm, prominent Golgi apparatus, several granules frequently localized in the Golgi area, and numerous mitochondria, often in cluster disposition. OKT4-positive cells show generally less condensed chromatin, scanty cytoplasm and poor organule pattern. Furthermore, positive lymphocytes with prominent nuclear irregularities are found in both lymphocytic subpopulations. Ultrastructural similarities between the T cell subsets phenotypically characterized by monoclonal antibodies and other immunological criteria are also discussed.