In addition to IL-2, IL-12 would constitute one of the most promising cytokines in the treatment of human neoplasms. IL-2 has been proven to induce in vitro and in vivo several evident changes in the secretion of cytokines and various other immunoinflammatory substances. In contrast, very little data are available about the immune effects of IL-12 in humans. The present study was carried out to investigate the in vivo immunoinflammatory effects of IL-12 by analyzing the secretions of neopterin, soluble IL-2 receptor (SIL-2R), tumor necrosis factor alpha (TNF), IL-2 and IL-6 in relation to the neuroendocrine function of the pineal gland, which is one of the most important organs involved in neuroimmunomodulation. Pineal endocrine function was investigated by evaluating the whole daily urinary excretion of the main catabolite of its hormone melatonin, 6-sulfatoxymelatonin (6-MTS). The study was performed on metastatic renal cell cancer patients. Each course of IL-12 consisted of 1.25 microg/ kg b.w. subcutaneously in the morning once a week for 3 consecutive weeks. The study evaluated 10 IL-12 courses. Mean serum levels of neopterin, SIL-2R and TNF significantly increased in response to IL-12, whereas no significant change occurred in IL-6 and IL-2 mean concentrations. Finally, 6-MTS urinary excretion was significantly reduced by IL-12 injection, particularly during the dark phase of the day. This preliminary study would suggest that IL-12 may induce important changes in the in vivo immunoinflammatory response. Moreover, IL-12 administration would suppress pineal endocrine activity, thus confirming its previously suggested involvement in the neuroimmunomodulatory processes. Because of the fundamental role of the pineal gland in neuroimmunomodulation, IL-12-induced immune variations could depend at least in part on its action at central neuroendocrine sites.