We have shown previously that interleukin-12 (IL-12) gene therapy induced strong antitumor effects in several syngeneic murine tumor models including 4T1 mammary adenocarcinoma. Antiangiogenic treatment with a monoclonal antibody (mAb) directed against the vascular endothelial growth factor receptor-2 (VEGFR-2) is another promising treatment approach that can cause transient suppression of tumor growth. We hypothesized that the combination of IL-12 gene therapy and anti-VEGFR-2 mAb will achieve better antitumor and antimetastatic effects against 4T1 adenocarcinoma than each treatment alone via implementation of different mechanisms. Administration of anti-VEGFR-2 mAb into BALB/c mice bearing s.c. 4T1 tumors induced significant suppression of tumor growth, as did intratumoral administration of naked IL-12 DNA. The combined treatment with anti-VEGFR-2 mAb and IL-12 DNA resulted in significantly enhanced inhibition of tumor growth as compared with each treatment alone. This combination was also effective against spontaneous lung metastases. In T-cell–deficient nude mice, both IL-12 DNA and anti-VEGFR-2 mAb were effective in suppressing tumor growth. In T-cell- and natural killer cell–deficient scid/beige mice, only anti-VEGFR-2 mAb was effective, suggesting that natural killer cells are involved in the antitumor effects induced by IL-12 DNA. In both types of immunodeficient mice, the combination of anti-VEGFR-2 mAb and IL-12 DNA was as effective in suppressing 4T1 tumor growth as anti-VEGFR-2 mAb alone. Antitumor effects of anti-VEGFR-2 mAb were associated with the inhibition of angiogenesis within the tumors, whereas the antiangiogenic effect of IL-12 gene therapy was not detected. Our results show a therapeutic benefit of combining IL-12 gene therapy and anti-VEGFR-2 mAb for cancer treatment.