Nitric oxide (NO) synthesis is strongly induced during interleukin (IL)-2 treatment of mice and humans. Although this free radical can act as a cytotoxic effector molecule against cancer cells, immunosuppressive effects have also been suggested. We evaluated the effects of curcumin on IL-2-induced NO synthesis and IL-2-induced antitumor responses in a mouse ascites tumor model. Curcumin inhibited inducible nitric oxide synthase (iNOS) expression and NO production, and thereby enhanced the proliferation and cytotoxic activity of cocultured lymphocytes and macrophages during IL-2 stimulation which we earlier established as an in vitro model of IL-2-induced NO synthesis. Curcumin also decreased apoptosis of cocultured lymphocytes and macrophages during IL-2 stimulation. In contrast, the curcumin-induced changes in proliferation and apoptosis were not observed in cultures of lymphocytes alone, macrophages alone, and cocultured lymphocytes/iNOS-knock out macrophages, all of which produced little nitrite during IL-2 stimulation. In conjunction with IL-2 treatment, oral curcumin administration significantly inhibited IL-2 therapy-induced urinary nitrite/nitrate excretion and iNOS expression of tumor tissues, and further increased the IL-2 therapy-induced prolongation of survival in a murine Meth-A ascites tumor model. Curcumin may be useful as an adjunct to increase the antitumor activity of IL-2 therapy.