The pineal hormone melatonin (MLT) has been shown to influence many biological functions, including immune response, cancer growth and brain neurotransmitter contents. On the basis of its biological properties, a study was started to evaluate the influence of MLT on IL-2 immunotherapy toxicity. The study was carried out in metastatic renal cancer patients. Thirty-three 5-day courses of IL-2 at a daily dose of 3 x 10(6) Cetus U/m2 were randomized to consist of IL-2 alone or IL-2 plus MLT (10 mg/day orally at 8.00 p.m.). The frequency of episodes of severe hypotension was significantly greater during IL-2 alone than during IL-2 + MLT. Moreover, the depressive symptomatology occurred more frequently during IL-2 alone. No other toxicity, including capillary leak syndrome, vomiting and fever, were significantly influenced by the concomitant treatment with MLT. These preliminary results would suggest that the pineal hormone MLT may be successfully associated with IL-2 in the immunotherapy of human tumors.