Nonspecific acid alpha-naphthyl acetate esterase (ANAE) activity has previously been evaluated with the sheep erythrocyte (E) rosette assay and found to be a useful T-cell marker in cell suspension studies. T-lymphocytes display a solitary red brown nodule of reaction product (T-pattern) which can be readily distinguished from the diffuse, cytoplasmic staining of monocytes (M-pattern). Freshly obtained human tissue blocks were fixed with buffered formal sucrose and maintained in Holt's syrup; cryostat sections were cut and incubated under conditions appropriate for the histochemical demonstration of lymphocyte acid alpha-naphthyl acetate esterase activity. The results clearly demonstrate that T-pattern ANAE+ lymphocytes populate the paracortical and interfollicular regions of normal lymph nodes and tonsils and the periarteriolar sheaths of normal spleen, T-cell zones, while nearly every germinal center (B-cell area) lymphocyte is ANAE-. Macrophages and dendritic reticulum cells display diffuse cytoplasmic ANAE activity (M-pattern). Nodular malignant lymphomas (B-cell origin) are ANAE-; an E+ diffuse, cutaneous lymphoma was ANAE+. The histochemical demonstration of ANAE activity may represent the first reproducible, routine technique to localize T-lymphocytes in tissue sections.