Rapamycin is an immunosuppressive agent routinely used in organ transplantation but also paradoxically exerts antiviral and antitumor activities. Pathogen-specific memory CD8(+) T-cell (T(CD8) ) responses were recently found to be augmented by rapamycin. However, whether rapamycin influences the magnitude and quality of anticancer T(CD8) responses is unknown. Importantly, how rapamycin may regulate simultaneous virus/tumor-specific and alloreactive T(CD8) in the same host remains unexplored. To answer these questions, we primed wild-type mice with allogeneic cells concomitantly expressing simian virus 40 large tumor antigen (T Ag), a viral oncoprotein with well-defined epitopes. Rapamycin selectively enhanced the cross-priming of T(CD8) specific for T Ag's most immunodominant epitope called site IV but not T(CD8) alloreactivity. Rapamycin-treated mice also had a high percentage of splenic CD127(high) KLRG1(low) T(CD8) and an increased frequency of site IV-specific T cells long after the peak of their primary response. When site IV was presented as a cytosolic minigene encoded by a recombinant vaccinia virus, rapamycin failed to boost the site IV-specific response. Therefore, the nature and presentation mode of antigen determine the susceptibility to the adjuvant effect of rapamycin. Our findings reveal the unexpected benefit of rapamycin treatment in recipients of allografts co-expressing tumor/viral Ags.