Cytochemical identification of T lymphocytes on the basis of alpha-naphthyl acetate esterase (NAE) activity was compared with immunologic markers for cell suspensions and/or cryostat sections of 113 specimens. Nonneoplastic tissues (peripheral blood, lymph nodes, spleens, tonsils, thymus, and pleural fluid) and specimens from various lymphoproliferative disorders, including acute and chronic lymphocytic leukemia, lymphosarcoma cell leukemia, hairy cell leukemia, non-Hodgkin's lymphomas of B-and T-cell types, and Hodgkin's disease, were evaluated. T (E-rosetting) cells demonstrated several patterns of NAE reactivity: 1) a strong globular reaction product, the most specific pattern for T-cell identification, 2) granular cytoplasmic staining, or 3) no reactivity. B lymphocytes revealed a granular pattern of NAE staining, were devoid of enzyme, or, in rare instances, exhibited strong NAE activity. Percentages of lymphoid cells with strong (globular) NAE activity closely paralleled T-cell (E-rosette) values in the majority of cases, with the best correlations observed for peripheral blood studies. However, discordant results were noted for some neoplastic and nonneoplastic tissues, including cases of T-cell lymphoma or leukemia. Markedly discrepant results were noted for thymic lymphocytes, most of which revealed E-rosette formation and weak or absent NAE activity. Lymph nodes involved by Hodgkin's disease demonstrated a heterogeneous pattern of staining in E-rosetting cells and in Reed-Sternberg variants. Cryostat section studies of reactive lymph nodes and nodular lymphomas demonstrated strong NAE staining in lymphoid cells of T-cell (interfollicular, internodular) areas, with little or no positivity in follicles or nodules (B-cell areas). NAE staining patterns further suggested that T cells comprise part of the follicular cuff and possibly represent a minor population of some neoplastic nodules. Although NAE determinations do not represent a consistently reliable alternative to immunologic methods for T-cell identification, this easily applicable cytochemical marker is complementary to other techniques in assessing neoplastic or nonneoplastic tissues, particularly cryostat sections. (Am J Pathol 97:17--42, 1979).