CD8+ memory T cells are critical in providing immunity to viral infection. Previous studies documented that antigen-specific CD8+ memory T cells are more resistant to radiation-induced apoptosis than naive T cells. Here, we determined the number and in vivo function of memory CD8+ T cells as immune reconstitution progressed following irradiation. Immediately following irradiation, the number of memory CD8+ T cells declined 80%. As reconstitution progressed, the number of memory cells reached a zenith at 33% of pre-irradiation levels, and was maintained for 120 days post-irradiation. In vitro, memory CD8+ T cells were able to produce cytokines at all times post-irradiation, but when adoptively transferred, they were not able to expand upon rechallenge immediately following irradiation, but regained this ability as reconstitution progressed. When proliferation was examined in vitro, irradiated memory CD8+ T cells were able to respond to mitogenic growth but were unable to divide.