Dendritic cell (DC)-based vaccines have exhibited minimal effectiveness in treating established tumors, likely because of factors present in the tumor microenvironment. One such factor is transforming growth factor β (TGF-β), a cytokine that is produced by numerous tumor types and has been demonstrated to impair DC functions in vitro. We have evaluated the effect of TGF-β on the immunostimulatory activities of DCs. We demonstrate that TGF-β exposure inhibits the ability of DCs to present antigen, stimulate tumor-sensitized T lymphocytes, and migrate to draining lymph nodes. Neutralization of TGF-β using the TGF-β-neutralizing monoclonal antibody 2G7 enhanced the ability of DC vaccines to inhibit the growth of established 4T1 murine mammary tumors. Treatment of 4T1 tumors transduced with the antisense TGF-β transgene (4T1-asT) with the combination of DC and 2G7 monoclonal antibody inhibited tumor growth and resulted in complete regression of tumors in 40% of the mice. These results demonstrate that neutralization of TGF-β in tumor-bearing mice enhances the efficacy of DC-based vaccines.