SUMMARYActivity of the enzyme acid α‐naphthyl acetate esterase (ANAE) in T lymphocytes and histiocytes in cutaneous tissue sections of patients with histologically confirmed lichen planus, chronic discoid eczema and mycosis fungoides was compared with results obtained using anin situimmunohistochemical technique employing an antiserum against the human T lymphocyte surface antigen (HTLA). In lichen planus and chronic discoid eczema, most of the cells in the cutaneous infiltrate exhibited both positive ANAE activity and the presence of HTLA. In mycosis fungoides, cells identified as T lymphocytes by the presence of HTLA often showed no ANAE activity. ANAE activity appears to be a useful marker of T lymphocytes in benign cutaneous infiltrates but is less reliable in labelling abnormal T cel