As a result of immunologic tolerance, the T cell response to self-tumor antigens is generally weak and insufficient to control tumor growth. Vaccination with mimotopes, or mimics of tumor antigens, is a promising strategy to improve immunotherapy. I hypothesized that optimal amino acid substitutions in a tumor antigen would create mimotopes that elicit effective antitumor immunity. To address this hypothesis, I screened peptide libraries for mimotopes of the tumor antigen AH1, the immunodominant antigen of the mouse colon carcinoma CT26. All of the identified mimotopes stimulated a tumor-specific T cell clone in vitro. To test these mimotopes in vivo, I developed a new vaccination technique using insect cells infected with baculoviruses encoding peptide-MHC molecules as both the source of antigen and the adjuvant. Vaccination with infected insect cells expressing some of the mimotopes identified in the library protected mice from subsequent tumor growth. However, vaccination with insect cells expressing the AH1 tumor antigen and other mimotopes identified in the library did not prevent tumor growth. I analyzed the frequency, effector function, and T cell receptor gene repertoire of the CD8⁺ T cells responding to mimotope vaccination to determine the characteristics that correlate with effective antitumor immunity. The protective mimotopes elicited more tumor-specific T cells that were more sensitive to stimulation with the AH1 peptide than the non-protective mimotopes. These T cells had a more restricted Vβ repertoire and expressed TCR molecules that were similar to those expressed by AH1-elicited T cells. Importantly, the T cells responding to the non-effective mimotopes did not inhibit the antitumor effects of the protective mimotopes. These results suggest that protective mimotopes generate antitumor immunity, in part, by enhancing the activation and differentiation of T cells that naturally respond to the tumor, not a new repertoire of T cells only elicited by the mimotope. T cells that prevent tumor growth have (1) high affinity for the tumor antigen, and recognize and eliminate tumor cells efficiently, or (2) high affinity for the mimotope, and expand and differentiate more readily after vaccination, or (3) high affinity for both the mimotope and the tumor antigen.