Rapid postnatal development of T-cell memory and loss of naive T-cell phenotype during aging differs between CD8 and CD4 T cells in the rhesus macaque. Representative FCM dot plots of naive (CD95^lowCD28^int./high; N), centralmemory (CD95^highCD28^high; CM), and effector-memory (CD95^highCD28^low; EM) T-cell phenotype distribution. Spleen cells were stained with FITC-conjugated CD3, PE-conjugated CD8β, or PerCP-conjugated CD4, PE- or CyC-conjugated CD28, and APC-conjugated CD95 mAbs. Gates were set on CD3⁺CD8⁺ or CD3⁺CD4⁺ cells as indicated. The percentage values represent the proportions of total CD8 and CD4 T-cells, respectively. In both CD4 and CD8 T-cell subsets, the percentage of naive cells declines profoundly during the early postnatal period, as demonstrated for a 2-year-old animal. Naive cells are almost completely depleted during advanced aging in both T-cell subsets, as illustrated by a representative profile from a 23-year-old animal. It is important to note that within the total CD95^high memory population, CD28^high CM cells predominate in CD4, whereas CD28^– EM cells predominate in CD8 T cells.