The hallmark of effective establishment of immune memory is the long-term memory cell that persists in the absence of antigen. To explore its characteristics, we investigated the differences between a resolved successful immune response, such as after influenza (flu) vaccination, and the state of chronic infection with persistent antigen, such as with cytomegalovirus (CMV), Epstein-Barr virus (EBV) or human immunodeficiency virus (HIV), which leads to defective T-cell memory. Immunophenotypic analyses using multi-parameter flow cytometry and tetramer technology identified a unique pattern of CD26(high) expression among influenza-specific CD8(+) T cells, but not among CD8(+) T cells specific for CMV, EBV (three different epitopes) or HIV. The median percentage of CD8(+) T cells expressing CD26 was 95.5% for influenza, but for cells specific for CMV, EBV and HIV it was 10.5%, 12%-19%, and 13.2%, respectively. These findings suggest that expression of CD26(high) may be a characteristic of a memory cell. CD26(high) expression correlates with expression of CD127, a marker of memory cells. Furthermore, CD26(high) cells can produce interleukin-2. These findings offer insight into the dynamics of T-cell differentiation, and they may offer a specific marker of a successfully developed memory CD8(+) T cell, that of CD26(high). This marker has the potential to be useful in studies of immune responses to infectious agents, and to new vaccine candidates.