In 4 patients with mycosis fungoides in the praemycotic, infiltrative and tumour stage of the disease, a differentiation and functional characterization of the infiltrate cells, obtained from cell suspensions and cutaneous smears, was carried out by means of enzyme cytochemical and immunological methods. The lymphocyte-associated acid esterase is considered to be a marker for mature T-cell populations. Apart from monocytes and histiocytes, acid esterase-positive small lymphocytoid cells with T-cell properties are found in the praemycotic stage. In the tumour stage, large lymphocytoid cells become increasingly prevalent, they show no acid esterase activity, but an intracytoplasmatic-localized reaction in the acid phosphatase activity. On the basis of the cytochemical pattern, it is assumed that these cells represent proliferating lymphoblasts.