We have used T-cell receptor (TCR) transgenic mice to analyze the interaction of tumors with the immune system. We show that the tumor cell line Lewis lung-lymphocytic choriomeningitis virus (LL-LCMV), genetically manipulated to express an H-2 D^b-restricted epitope of the lymphocytic choriomeningitis virus glycoprotein (LCMV_33–41), cna grow progressively in TCR transgenic mice, where ∼50% of CD8+ T cells are specific for LCMV_33–41. TCR transgenic T cells were not deleted in tumor-bearing mice, and their surface phenotype and cytokine secretion patterns remained typical of naive T cells. Also, TCR transgenic T cells from tumor-bearing mice had undiminished capacity to proliferate to antigen in vitro. Tumor-protective immune responses could be elicited in TCR transgenic mice by immunization with LCMV_33–41 peptide-loaded dendritic cells. Tumor resistance correlated with the switch of TCR transgenic T cells from a CD44^low to a CD44^high phenotype and increased capacity to produce IFNγ in vitro. Results similar to those obtained in TCR transgenic mice were also obtained using an adoptive transfer system, where small numbers of TCR transgenic T cells were injected into normal C57BL/6 hosts. These data indicate that even large tumors may not induce specific immunization, tolerance, or anergy to tumor antigens, and that high numbers of tumor-specific CTL precursors are not sufficient to provide tumor resistance.