Tumor cell-induced angiogenesis, i.e., new blood vessel formation within tumor tissue, is an essential requirement for the growth of solid neoplasms. Interleukin-12 (IL-12) inhibits growth of a variety of experimental tumors in vivo. We tested whether antitumor activity of IL-12 is related to the inhibition of angiogenesis induced by tumor cell lines. Angiogenesis was induced in x-ray immunosuppressed Balb/c mice by intradermal injection of the following human tumor cells: T47D, originating from mammary carcinoma; A431, derived from vulval carcinoma; and Skv, established from bowenoid papulosis, Systemic treatment of the mice with murine IL-12 significantly decreased angiogenesis induced by human tumor cells in a time-and dose-dependent manner. Preincubation of human cells in vitro with IL-12 did not inhibit tumor cell-induced angiogenesis, suggesting that the antiangiogenic capacity of IL-12 is restricted to in vivo conditions. Treatment of the mice with rat antibody against murine interferon-gamma (IFN-gamma) resulted in counteracting the antiangiogenic effect of murine IL-12. Furthermore, human IFN-gamma inhibited the angiogenic activity of human tumor cell lines. This indicates that IFN-gamma is a mediator of the antiangiogenic effect of IL-12. The results show that the mechanism of antitumor action of IL-12 may depend not only on the immunostimulatory activity of this cytokine but also on its effect on tumor cell-induced angiogenesis. IL-12 should be considered as a potential candidate for the treatment of angiogenesis-dependent malignancies.