Increasing the efficacy of adoptively transferred, tumor antigen specific T cells is a major goal of immunotherapy. Clearly, a more thorough understanding of the effector phase of T cell responses, within the tumor site itself, would be beneficial. To examine this issue, we adoptively transferred tumor antigen-specific effector T cells into tumor-bearing mice, then performed kinetic evaluations of their phenotype, function, and survival in tumors, draining lymph nodes (dLNs), and spleens during regression of murine fibrosarcomas. Effector function in tumors was quantitated through the use of a novel intratumoral cytolytic assay. This approach revealed dynamic changes in the phenotype, cytolytic capacity, and viability of tumor infiltrating effector T cells during the course of tumor regression. Over a period of days, T cells within tumors rapidly transitioned from a CD25hi/CD27hi to a CD25low/CD27low phenotype and displayed an increase in cytolytic capacity, indicative of effector maturation. Simultaneously, however, the viability of maturing T cells within tumors diminished. In contrast, transferred T cells trafficking through lymphoid organs were much more static, as they maintained a stable phenotype, robust cytolytic activity, and high viability. Therefore, there exists a marked phenotypic and functional divergence between tumor-infiltrating effector T cells and their counterparts in lymphoid organs. Our results indicate that the population of tumor-infiltrating T cells is unique in experiencing rapid effector maturation post-transfer, and suggest that strategies aimed at prolonging the survival of CD25low/CD27low full effectors, which displayed the highest levels of intratumoral cytolytic activity, should enhance the efficacy of T cell based tumor immunotherapies. © 2007 Norian, Allen.