It has been generally believed that human CD8+ memory cells are principally found within the CD45RO high population. There are high frequencies of CD8+ memory CTL specific for the human CMV tegument phosphoprotein pp65 in PBMC of long-term virus carriers; the large population of memory CTL specific for a given pp65 peptide contains individual CTL clones that have greatly expanded. In this study, we found high frequencies of pp65 peptide-specific memory CTL precursors in the CD45RO high CD45RA− population, but also appreciable frequencies in the CD45RA high subpopulation. Because the majority of CD8+ T cells in PBMC are CD45RA high, more of the total pp65-specific memory CTL pool is within the CD45RA high than in the CD45RO high compartment. Using clonotypic oligonucleotide probes to quantify the size of individual pp65-specific CTL clones in vivo, we found the CD45RA high population contributed 6-to 10-fold more than the CD45RO high population to the total virus-specific clone size in CD8+ cells. During primary CMV infection, an individual virus-specific CTL clone was initially CD45RO high, but after resolution of infection this clone was detected in both the CD45RO high and the CD45RA high populations. We conclude that CD45RA+ human CD8+ T cells do not solely comprise naive cells, but contain a very significant proportion of memory cells, which can revert from the CD45RO high to CD45RA high phenotype in vivo.