Flow cytometric analysis of major lymphocyte populations and their subsets reveals age-related changes in the cellular human immune system. Immunophenotypic markers were evaluated in 110 normal pediatric subjects, divided into groups of newborn infants, infants aged 2 days to 11 months, and children aged 1 to 6 years and 7 to 17 years; results were then compared with those obtained from 101 normal adults aged 18 to 70 years. Comparisons among age groups from newborn infants through adults reveal progressive declines in the absolute numbers of leukocytes, total lymphocytes, and T, B, and natural killer (NK) cells. The percentages of T cells within the total lymphocyte population increase with age, in both CD4+ and CD8+ subsets. Percentages of B and NK cells are higher in newborn infants than in adults. The expression of the activation markers interleukin-2R and HLA-DR on T cells increases with age, as does the NK-associated expression of CD57 on CD8 cells. The proportions of B lymphocytes that coexpress CD5 or CDw78 decrease with age, whereas expression of Leu-8 and CD23 increases. The proportion of CD4 cells bearing the CD45RA and Leu-8 markers is consistently lower in adults than in children. These data may serve as a reference range for studies of pediatric subjects.