We report the phenotypical and functional characterization of a cord blood mononuclear lymphocyte subset (CBMC) which expresses the p55 chain of interleukin-2 receptor (IL-2R, CD25). The great majority of CD25-positive CBMC are activated CD3-positive T lymphocytes in that most of them are HLA-DR-positive, coexpress both CD4 and CD8 antigens and at least a part of them display high affinity IL-2R. In keeping with the expression of both chains of IL-2R (p55 and p75), enriched CD25-positive CBMC are able to display strong IL-2-induced proliferation and lymphokine-activated killing activity. The percentage of CD25-positive cells is higher in cord blood than in adult peripheral blood and their presence could be peculiarly important in the neonatal period, when specific T-cell-mediated immune responses are still on their way to full maturation.