We studied the ability of CD8+ T cells activated in vitro to mediate tumor protection after transfer into adoptive hosts. TCR transgenic CD8+ T cells were activated in culture with DC and specific peptide antigen, and briefly expanded in IL-2 containing medium. Cultured cells acquired a CD44hiCD62Llo phenotype, and following in vivo transfer they preferentially homed to non-lymphoid tissues and spleen. In vivo, their numbers declined between day 0 and day 20, and then remained relatively stable from day 20 to day 90. Over time, many of the injected cells re-expressed CD62L, and acquired the ability to localize to secondary lymphoid organs. Transferred T cells underwent low-level proliferation, expressed IL-7Ralpha and IL-15Rbeta, were cytotoxic in vivo, and retained the ability to produce IL-2, IFN-gamma and TNF-alpha upon ex vivo restimulation. In addition, transferred T cells conferred a high degree of tumor protection, which was greatest immediately after injection, and remained significant even when tumor was given 90 days after T-cell transfer. We conclude that in vitro generated effector T cells can mediate immediate and long-term tumor protection, and develop into long-lived memory T-cell populations in vivo.