Programmed death-1 (PD-1) regulates T cell exhaustion during chronic infections. Blocking the PD-1:PD-ligand (PD-L) pathway reinvigorates exhausted CD8 T cells. Exactly how blocking PD-1:PD-L interactions improves T cell immunity, however, remains unclear. PD-1:PD-L blockade could reprogram all exhausted T cells to become antiviral effectors. Alternatively, this blockade might selectively expand a subset of exhausted T cells. We have identified two subpopulations of exhausted CD8 T cells during chronic viral infection in mice. One subset of exhausted CD8 T cells is rescued by αPD-L1 blockade, whereas the other subset appears more terminally differentiated and responds poorly to PD-1:PD-L blockade. Blocking PD-1:PD-L interactions reduces spontaneous apoptosis and enhances expansion and protective immunity of the rescuable subset, but not the more terminally differentiated subset of exhausted CD8 T cells. These results have implications for predicting clinical responses to PD-1-based therapeutic interventions and for understanding T cell dynamics during persisting infections.