Although the recently identified ICOS/B7h costimulatory counterreceptors are critical regulators of CD4+ T cell responses, their ability to regulate CD8+ responses is unclear. Here we report using a tumor-rejection model that ectopic B7h expression can costimulate rejection by CD8+ T cells in the absence of CD4+ T cells. Although responses of naive T cells were significantly augmented by priming with B7h, B7h was surprisingly effective in mobilizing recall responses of adoptively transferred T cells. To explore why secondary responses of CD8+ T cells were particularly enhanced by B7h, kinetics of ICOS up-regulation, proliferative responses, and cytokine production were compared from both naive and rechallenged 2C-transgenic T cells costimulated in vitro. Although B7h costimulated proliferative responses from both CD8+ populations, rechallenged cells were preferentially costimulated for IL-2 and IFN-γ production. These results indicate that ICOS/B7h counterreceptors likely function in vivo to enhance secondary responses by CD8+ T cells.