Lymphocytes of peripheral blood, bone marrow, spleen, vermiform appendix, tonsil and Mantoux skin reaction were examined by electron microscopy (EM) and classified as T, B and Null cells by E-rosette and immunoglobulin membrane-receptor characteristics. The low pH and ionic strength of the fixative solution for EM, and some other minor procedural modifications, made it possible to distinguish B and T lymphocytes morphologically. T-cells have electron-dense cytoplasm and euchromatin in the nucleus whereas B-cells constantly have electron-lucent cytoplasm and euchromatin in the nucleus. A proportion of lymphocytes were unclassifiable by their ultrastructural features. These unclassifiable cells may be Null cells as determined by the classical techniques. The specificity and simplicity of this EM technique for T and B lymphocytes is especially useful for studies of immunocompetent-cell topography and cell-to-cell interaction in lymphoid organs. It may also be utilized for diagnostic purposes in immunocytic dyscrasias.