Thirty-two cord blood samples taken after caesarean section or vaginal delivery and concurrent venous blood samples obtained from normal adult controls were evaluated using monoclonal antibodies. The percentage of circulating pan-T-cell+ lymphocytes was significantly lower in cord blood (46%) compared with adult controls (72%). In the cord cells, 22% showed reactivity with the common thymocyte antigen compared with less than 1% in adult controls. The helper:suppressor ratio was lower in cord blood (1.71) compared with 1.98 for adult blood. These figures reflect a unique population (12%) of immature T cells in cord blood that coexpress helper and suppressor phenotypes. These features are not found in adult blood. These double-labeling studies characterized a previously undescribed blood T-cell phenotype which correlates negatively with gestational age (R = -0.93). These studies reveal the presence of an immature population of T cells in normal human neonatal blood that exhibit the phenotype characteristic of normal developing thymocytes.