Plasma samples were collected from 20 patients undergoing phase I clinical trial with flavone-8-acetic acid (FAA; 4.8 g m-2 per dose) in combination with recombinant human interleukin-2 (rhIL-2; 6-18 i.u. m-2 per day) for the treatment of metastatic melanoma. Samples were analysed for nitrate content as an indication of the oxidation of L-arginine to nitric oxide. Pretreatment plasma nitrate levels (53 +/- 4 microM) were significantly above those of healthy volunteers (19 +/- 4 microM). The maximum plasma nitrate concentration obtained after treatment, 190 +/- 29 microM (range 49 to 655 microM), was comparable to that of mice treated with FAA. Most of the increases occurred 3-5 days after initiation of a 5 day infusion of rhIL-2, but three of the increases occurred within 2 days of a 1 h infusion of FAA alone. The maximum plasma nitrate concentrations of the three patients which underwent remission (two complete, one partial) following treatment (368 +/- 143 microM) were significantly higher (P < 0.05) than those of patients with progressive disease. Hypotension was the major dose-limiting side effect, and there was no relationship between the degree of hypotension and the rise in plasma nitrate. The results provide evidence that treatment of patients with FAA and rhIL-2 induce the synthesis of nitric oxide, a physiological mediator and potential cytotoxic agent.