BACKGROUND In previous animal studies, interleukin 12 (IL 12) was shown to inhibit the growth of a wide spectrum of tumors in vivo but to have no direct effect on tumor cells in vitro. Also, contrary to the expectation of a T-cell-mediated effect, the antitumor activity of IL 12 was not completely abrogated in tests of T-cell-deficient mice. These observations suggest that IL 12 may possess antiangiogenic properties that account for its tumor-inhibitory effects in vivo. OBJECTIVE Our goal was to investigate the hypothesis that IL 12 has antiangiogenic properties. METHODS A model of basic fibroblast growth factor-induced corneal neovascularization in mice was used to evaluate the effects of IL 12 and interferon gamma (IFN gamma) on angiogenesis in vivo. Different strains of male mice, e.g., immunocompetent C57BL/6 mice, severe combined immune-deficient (SCID) mice, natural killer cell-deficient beige mice, and T-cell-deficient nude mice, were treated with IL 12 (1 microgram/day) intraperitoneally for 5 consecutive days. The extent of neovascularization in response to a basic fibroblast growth factor pellet and the inhibition of neovascularization by IL 12 or IFN gamma were assessed by measuring the maximal vessel length and the corneal circumference involved in new blood vessel formation. The antitumor activities of IL 12 and of the angiogenesis inhibitor AGM-1470 were evaluated in Lewis lung carcinoma-bearing mice. In vitro proliferation studies were performed on bovine capillary endothelial cells, mouse pancreatic islet endothelial cells, and mouse hemangioendothelioma cells. RESULTS IL 12 treatment almost completely inhibited corneal neovascularization in C57BL/6, SCID, and beige mice. This potent suppression of angiogenesis was prevented by the administration of IFN gamma-neutralizing antibodies, suggesting that the suppression was mediated through IFN gamma. In addition, the administration of IFN gamma reproduced the antiangiogenic effects observed during treatment with IL 12. Treatment with IL 12 and AGM-1470 combined did not increase toxicity and showed a trend toward enhanced antitumor efficacy in Lewis lung carcinoma-bearing mice. CONCLUSIONS IL 12 strongly inhibits neovascularization. This effect is not mediated by a specific cell type of the immune system. Instead, IL 12 has been shown to induce IFN gamma, which, in turn, appears to play a critical role as a mediator of the antiangiogenic effects of IL 12. CONCLUSIONS Recognition of the mechanisms of the antiangiogenic properties of IL 12 may be crucial in planning its clinical applications, including a possibility of coadministration with other inhibitors of neovascularization.