Skin lesions from 14 patients with mycosis fungoides (MF) were studied immunohistochemically with monoclonal antibodies that identify pan T (Leu4), T-helper/inducer (TH), T-suppressor/cytotoxic (Ts) (Leu3a, Leu2a) and pan B (Leu12) cells. Lymphoid cell reactivity was diverse among these cases. Among 12 cases of MF, TH cells were predominant over Ts cells, and one case featured nonreactive Ts cells in the dermis. In one case of MF, Ts cells were predominant. One case of MF showed strong reactivity for both T cell subset markers. The intraepidermal lymphoid cellularity was Leu4-reactive in 14 cases of MF. Among these 14 cases, the intraepidermal cellularity was Leu3a-reactive in 11 and Leu2a-reactive in 4. The use of such studies of T cell subsets may demonstrate a correlation of MF with cytomorphology, clinical stage, and disease prognosis.