The combination of cisplatin-based chemotherapy with interleukin-2 (IL-2) and interferon-alpha (IFN-alpha), referred to as biochemotherapy, has produced overall response rates of greater than 50% in advanced melanoma patients, with durable complete responses in the range of 5-10%. The mechanism of action of biochemotherapy is unknown. Preclinical work suggests synergistic interactions between the cytotoxic agents, especially cisplatin, and the biological agents in killing melanoma cells. Immune effector cells activated by the components of the biochemotherapy may also be involved, as direct cytotoxic effectors and/or as sources of secondary cytokines, which can induce nitric oxide (NO) production in a wide variety of cell types. In addition, high levels of neopterin, a marker of monocyte/macrophage activation, have been found in patients undergoing immunotherapy or biochemotherapy for melanoma. Based on these data, we hypothesized that the degree of elevation of serum NO metabolic products and neopterin during treatment would correlate with the response to biochemotherapy in melanoma patients. Blood samples were obtained before and during preoperative biochemotherapy with cisplatin, vinblastine, dacarbazine, IL-2 and IFN-alpha in 45 melanoma patients with locoregionally advanced disease. NO was measured as nitrite after enzymatic reduction, using the colorimetric assay of Griess, and neopterin was measured by radioimmunoassay. Our results demonstrate a higher day 5 nitrite level (of borderline statistical significance, P = 0.057) in major responders to the therapy than in those who did not achieve a major response, while there was no difference in the elevation in neopterin level during therapy between major and non-major responders. These results suggest that induction of NO during biochemotherapy may be playing a role in the mechanism of action of this therapy, while the role of monocyte/macrophage activation is still in question.