Proliferation of postthymic T cells in neonates and young adults. Lymphocytes were isolated from 39 cord blood samples from newborns of 26 to 41 weeks gestation and 15 healthy young adults. T cells in the cell cycle were identified by the expression of the Ki67 nuclear antigen by flow cytometric analysis of CD4⁺CD45RO^– (naive) and CD8⁺CD45RO^– T cells. Results in the adults (open boxes) and the full-term infants (shaded boxes) are shown as box plots. Frequencies of Ki67-expressing naive T cells were 10-fold higher in the infants than in the adults. The CD8⁺ T-cell subset included a higher proportion of Ki67⁺ cells in neonates (P < .001) and adults (P = .006) compared with CD4⁺ T cells (A). Frequencies of Ki67⁺ cells within the CD4⁺CD45RO^– (open circles) and CD8⁺CD45RO^– cells (filled circles) T-cell populations are given in correlation to the gestational ages of the neonates. Proliferation rates of cord blood CD4⁺ and CD8⁺ T cells were highest during the early phase of the third trimester and exponentially declined to maturity (R = 0.81 for CD4⁺ T cells; R = 0.79 for CD8⁺ T cells). The number of cycling cells remained elevated in full-term infants compared with adults. Frequencies of Ki67⁺CD45RO^–CD8⁺ T cells were always higher than in their CD4⁺ counterparts (B). The ratio of CD4⁺CD45RO^– and CD8⁺CD45RO^– T cells was determined by flow cytometry in lymphocytes isolated from cord blood or peripheral blood of young adults. CD4⁺ T cells outnumbered CD8⁺ T cells in newborns and adults, but the ratios were significantly different (P < .001), with a relative reduction of CD4⁺ T cells over the first 2 decades of life (C).