Using HLA class I-viral epitope tetramers to monitor herpes virus-specific CD8+ T cell responses in humans, we have shown that a significant fraction of responding cells revert from a CD45RO+ to a CD45RA+ state after priming. All tetramer-binding CD45RA+ cells, regardless of epitope specificity, expressed a phenotype LFA-1 high CCR7 low that was stable for at least 10 years in infectious mononucleosis patients and indefinitely in asymptomatic carriers. CD8+ CD45RA+ LFA-1 high cells were not present in cord blood but in adults account for up to 50% of CD8+ CD45RA+ cells. These CD45RA+ LFA-1 high cells have significantly shorter telomeres than CD45RA+ LFA-1 low cells, suggesting that the latter represent a naive population, while the former are memory cells. CD45RA+ memory cells are a stable population of noncycling cells, but on stimulation they are potent producers of IFN-γ, while naive CD8+ cells produce only IL-2. The chemokine receptor profile and migratory potential of CD45RA+ memory cells is very similar to CD45RO+ cells but different to naive CD8 cells. In accord with this, CD45RA+ memory cells were significantly underrepresented in lymph nodes, but account for virtually all CD8+ CD45RA+ T cells in peripheral tissues of the same individuals.