In this report, we have studied the immunogenicity of the nominal antigen, carcinoembryonic antigen (CEA), and that of an anti-idiotype antibody, 3H1, which mimics CEA and can be used as a surrogate for CEA. We have demonstrated that immunization of CEA transgenic mice with bone marrow-derived mature dendritic cells (DC) loaded with anti-idiotype 3H1 or CEA could reverse CEA unresponsiveness and result in the induction of CEA-specific immune responses and the rejection of CEA-transfected MC-38 colon carcinoma cells, C15. Immunized mice splenocytes proliferated in an antigen-specific manner by a mechanism dependent on the functions of CD4, MHC II, B7-2, CD40, CD28, and CD25. However, immune splenic lymphocytes isolated from 3H1-DC-vaccinated mice when stimulated in vitro with 3H1 or CEA secreted significantly higher levels of Th1 cytokines than did CEA-DC vaccinated mice. DC vaccination also induced antigen-specific effector CD8+ T cells capable of expressing interluekin-2, IFN-γ, and tumor necrosis factor (TNF)-α and displayed cytotoxic activity against C15 cells in an MHC class I-restricted manner. 3H1-DC vaccination resulted in augmented CTL responses and the elevated expression of CD69, CD25, and CD28 on CD8+ CTLs. The immune responses developed in 3H1-DC-immunized mice resulted in rejection of C15 tumor cells in nearly 100% of experimental mice, whereas only 40% of experimental mice immunized with CEA-DC were protected from C15 tumor growth. These findings suggest that under the experimental conditions used, 3H1-DC vaccination was better than CEA-DC vaccination in breaking immune tolerance to CEA and inducing protective antitumor immune responses in this murine model transgenic for human CEA.