Experimental work has shown that, during the development of tumours, host macrophages are triggered to produce high levels of prostaglandin E2 which inactivates natural killer cells and suppresses lymphokine activated killer cell development. Sustained, uninterrupted indomethacin treatment, when combined with interleukin-2 (IL-2), can totally eradicate experimental metastases. Most trials utilizing high dose IL-2 employ indomethacin and ranitidine in order to alleviate or prevent IL-2 toxicity, but only administer these medications concurrently with IL-2 therapy. A Phase II trial was conducted in patients with advanced melanoma. Patients received 50-75 mg indomethacin three times daily and 150 mg ranitidine twice daily, starting at least 1 week prior to IL-2 and continuing until intolerance or disease progression. Continuous venous infusion IL-2 was administered for three courses, each consisting of 5 days of treatment at 18 x 10(6) iu/m2/day for the first course with escalation of dose for the subsequent courses if toxicity allowed. Twenty-one patients were eligible to receive all components of therapy. Three patients achieved an objective response (one complete and two partial), giving a response rate of 14%. However, two of these objective responses (one complete and one partial) were achieved on indomethacin and ranitidine alone, prior to the commencement of IL-2 therapy. In this study, indomethacin and ranitidine, without IL-2, have been shown to have antitumour activity in advanced melanoma; continuous infusion IL-2 appeared to add little to the response seen with these two agents.