Cell-mediated immunity using the E-rosette technique, the graft-versus-host reaction and intradermal skin tests were investigated in two groups of patients, twenty-two with benign lymphoproliferative diseases of the skin (lymphocytic infiltration of the skin and lymphadenosis benigna cutis) and nine with malignant lymphoproliferative diseases (mycosis fungoides and lymphoma cutis). The effect of thymic hormone upon T lymphocytes which proved to have impaired functional capacity was investigated. Seventeen of the twenty-two patients with benign lymphoproliferative diseases were found to have intact cellular immunity whereas seven of the nine patients with malignant disease proved to be immunodeficient. The determination of immune competence may be of help in establishing the correct diagnosis in doubtful cases and might also be useful in the follow-up of these patients. The improvement in functional capacity of T lymphocytes resulting from their interaction with a thymic hormone indicates that it holds therapeutic promise.