The mechanisms responsible for the immunostimulatory role of the pineal hormone melatonin (MLT) are still obscure. To investigate the influence of MLT on interleukin-2 (IL-2)-induced immune effects in cancer, we compared the results obtained in 14 cancer patients treated with IL-2 (6 x 106 IU/day s.c. for 5 days/week for 4 weeks) plus MLT (10 mg/day orally) with those seen in 14 patients treated with IL-2 alone and with those obtained from 14 other patients treated with MLT only. All patients were affected by metastatic solid neoplasms. The increase in the mean number of lymphocytes, T lymphocytes, natural killer cells, CD25-positive cells and eosinophils was significantly higher in patients treated with IL-2 plus MLT than in those receiving IL-2 alone. On the contrary, the increase in mean serum levels of the macrophage marker neo-pterin was significantly higher in patients treated with IL-2 alone than in those treated with IL-2 plus MLT. Finally, MLT alone has no significant effect on immune cell mean number and on neopterin secretion. These results would suggest that the immunostimulatory action of M LT requires the concomitant presence of IL-2 and that two of the main target cells for MLT activity in humans are represented by T helper lymphocytes of type 2, which are involved in IL-2-induced eosinophilia by the release of IL-5, and macrophages, which may inhibit IL-2-dependent immune functions.