UV-inactivated, replication-defective Sendai virus particles [hemagglutinating virus of Japan envelope (HVJ-E)] injected into murine colon carcinoma (CT26) tumors growing in syngeneic BALB/c mice eradicated 60% to 80% of the tumors and obviously inhibited the growth of the remainder. Induced adaptive antitumor immune responses were dominant in the tumor eradication process because the effect was abrogated in severe combined immunodeficient mice. Murine and human dendritic cells underwent dose-dependent maturation by HVJ-E in vitro. Profiles of cytokines secreted by dendritic cells after HVJ-E stimulation showed that the amount of interleukin-6 (IL-6) released was comparable to that elicited by live HVJ. Real-time reverse transcription-PCR and immunohistochemistry revealed that HVJ-E induced a remarkable infiltration of dendritic cells and CD4⁺ and CD8⁺ T cells into tumors. In addition, CT26-specific CTLs were induced with the evidence of enhanced CD8⁺ T-cell activation in a CD4⁺CD25⁻ T cell–dependent manner. On the other hand, conditioned medium from dendritic cells stimulated by HVJ-E rescued CD4⁺CD25⁻ effector T-cell proliferation from Foxp3⁺CD4⁺CD25⁺ regulatory T cell (Treg)–mediated suppression and IL-6 was presumably dominant for this phenomenon. We also confirmed such rescue in mice treated with HVJ-E in vivo. Moreover, antitumor effect of HVJ-E was significantly reduced by an in vivo blockade of IL-6 signaling. This is the first report to show that HVJ-E alone can eradicate tumors and the mechanism through which it induces antitumor immune responses. Because it can enhance antitumor immunity and simultaneously remove Treg-mediated suppression, HVJ-E shows promise as a novel therapeutic for cancer immunotherapy. [Cancer Res 2007;67(1):227–36]