Circulating neopterin is derived from monocytes and/or macrophages that produce it upon stimulation by interferon-gamma released from activated T cells. Neopterin production has been proposed as a marker of biological response in the clinical administration of a number of cytokines. Changes in neopterin production as indicated by urinary neopterin excretion were studied in four patients with ovarian carcinoma receiving intraperitoneal interleukin-2 and lymphokine-activated killer cells. Neopterin production increased approximately threefold during treatment with interleukin-2 at doses which represent or exceed the maximum tolerated dose by this route of administration. Increased neopterin apparently was derived from systemic, not regional, tissues. The physiologic role(s) of pterins in immune responses is uncertain. In an in vitro system, the presence of neopterin or tetrahydrobiopterin or the pterin synthesis inhibitor, N-acetyl serotonin, did not modulate cytotoxic effects of lymphokine-activated killer cells.