Surface phenotype of peripheral blood lymphocytes (PBL) from preterm infants was evaluated using monoclonal antibodies which define T cell membrane antigens associated with processes of maturation and activation of these cells. In the majority of preterm infants born during the 25th and 26th week of gestation, PBL included higher percentages of cells bearing an immature/activated surface phenotype characterized by the presence of CD1, CD38, and CD71 surface antigens than in term newborns. In these gestational age groups, PBL included, in particular, very high percentages of lymphocytes (range: 22–60) expressing the p55 chain of interleukin-2 receptor (IL-2R). After the 26th week of gestation, PBL of some preterm neonates included, as well, high percentages of lymphocytes bearing immature/activated phenotype; their median values, however, were not significantly different from those observed in term newborns. Our data suggest that the presence of the p55 chain of IL-2R on the surface of neonatal lymphocytes could be correlated with the immaturity of these cells.