Immunotherapy has been proposed as an alternative to conventional cancer therapies due to its reduced toxicity and ability to induce long-lasting anti-tumor immune responses. Dendritic cell (DC) vaccination is one immune-based anti-cancer strategy that has received attention due to the ability of DC to process and present antigen to T lymphocytes to initiate immune responses. However, the clinical efficacy of DC-based immunotherapy against established cancers in humans has been extremely low and despite recent advances, objective response rates in DC vaccine trials are rarely above 10%. This lack of efficacy is due in part to immunosuppressive factors, such as transforming growth factor &beta (TGF-&beta), present in the tumor microenvironment that promote tumor immune escape. Therefore, TGF-&beta represents a major barrier to effective cancer immunotherapy and strategies to neutralize this cytokine may lead to more efficacious DC vaccines.In this study, we employed two small molecule transforming growth factor &beta receptor type I (T&betaRI/ALK5) kinase inhibitors (HTS466284 and SM16) in combination with DC vaccines to treat established TGF-&beta-secreting 4T1 mammary tumors. The results demonstrate that while both inhibitors blocked the effects of TGF-&beta in vitro, HTS466284 by itself or in combination with DC vaccination was unable to consistently control the growth and metastasis of established 4T1 tumors. In contrast, SM16 inhibited the growth of established tumors when delivered orally and suppressed the formation of pulmonary metastases when delivered orally or via daily intraperitoneal (i.p.) injection. The efficacy of SM16 was dependent on cellular immunity as this drug had no effect in immunodeficient SCID mice. Furthermore, orally delivered SM16 in combination with DC vaccination led to complete tumor regression in several mice that correlated with increased T cell infiltration of the primary tumor and enhanced in vitro IFN-gamma production and tumor-specific cytolytic activity by splenocytes. Finally, a suboptimal dose of SM16 that failed to control primary tumor growth on its own synergized with DC vaccination to inhibit the growth of established 4T1 tumors. These findings suggest that blockade of TGF-&beta signaling using a small molecule T&betaRI/ALK5 kinase antagonist may be an effective strategy to bolster the efficacy of DC-based cancer vaccines.