AbstractThe development of monoclonal antibodies to cell‐surface antigens has provided method for characterizing distinct subpopulations of T‐cells. In the present study we have quantified peripheral blood T‐cell subpopulations in premature infants born weighing less than 1,500 g (1123 ± 223 g) and ranging in gestational age from 25 to 32 weeks. The relative proportion of T4 cells in the very low birth weight (VLBW) infants was markedly higher at l week and 1 month of age (mean ± SEM; 67.5 ± 4.1 and 59.2 ± 1.6) than in adult controls (47.2 ± 1.5). The percentage of T4 cells remained elevated until 6 months of age, when it decreased to a level comparable to that in adults. In contrast, the proportion of T8 cells was significantly lower than the adult level at 1 week and 1 month of age. The T4/T8 ratio in the VLBW infants was higher at 1 week (4.3 ± 0.5) and 1 month (3.5 ± 0.2) than in adult controls (2.0 ± 0.1). Thereafter, the T4/T8 ratio decreased but was still significantly higher than that in adult controls at 6 months of age (2.6 ± 0.2). The absolute numbers of total T‐cells (T3) and T8 and T4 cells were significantly higher in VLBW infants. The numbers of T8 cells were significantly lower in the first month of life than at 3‐6 months of age. These alterations in the T‐cell subsets in the first 6 months of life suggest that postnatal T‐cell phenotypic changes in VLBW infants may parallel the T‐cell ontogenetic process which occurs during the last trimester of pregn