E rosette-forming (T) lymphocytes and surface immunoglobulin-bearing lymphocytes were estimated in 85 patients with malignant melanoma. The melanoma patient group had lower mean levels of T lymphocytes and higher mean levels of immunoglobulin-bearing (? B) lymphocytes than did normal subjects. The absolute and percentage depressions of T-cell levels in the melanoma patients were stage-related, as was the depression of total lymphocyte and B-lymphocyte levels. The T lymphopenia in the melanoma patients could, in vitro, be partially abolished by fetal calf serum (as used in many E rosetting methods), and could be totally abolished by thymosin fraction 5 (Hoffmann-La Roche) at optimum concentration. In view of the ability of thymosin to restore T cells to normal levels in all of the T-lymphopenic patients, a clinical trial of this hormone in selected melanoma patients of all stages appears to be warranted.