Functional T-cell subpopulations have been evaluated in the peripheral blood of 124 melanoma patients (71 non-metastatic and 53 metastatic) using monoclonal antibodies: OKT3, OKT4, OKT8, every 2 months for 1 year. The levels of OKT3+ cells were significantly lower in metastatic patients than in normal controls and they decreased in the advanced phases of the disease. Percentages and absolute OKT4+ cell values were reduced in metastatic patients only, while a significant reduction in OKT8+ cells was noted in both long-surviving, non-metastatic and metastatic patients. The ratio of OKT4+/OKT8+ cells was increased in non-metastatic patients and in patients remaining metastasis-free for 4 years after resection of their metastases. Patients with prolonged survival show normal T-helper cells and low T-suppressor cells with a significant increased ratio OKT4/OKT8. In the patients who developed visceral metastases and in those who died the progressive reduction in total T cells is mainly due to the decrease in OKT4+ cells.