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Low alloreactivity of umbilical cord blood (UCB) T-cells may explain diminished graft-versus-host-disease after UCB transplantation. We investigated whether UCB T-cells have an intrinsic lower capacity to become activated. T-cells from UCB or adult blood (AB) were stimulated with anti-CD3 and anti-CD28 antibodies. On days 1-3 after stimulation, T-cell activation was determined by CD25 expression, proliferation was measured, and kinetics of cell division were analyzed by staining with CFSE. UCB and AB T cells exhibited similar numbers of activated and proliferating cells, but the extent of activation was lower in UCB T-cells. Enzyme-linked immunospot analysis showed lower levels and slower kinetics of IL-2, IL-4, IL-10, and IFN-gamma secreting cells for UCB T-cells. Comparison of UCB T-cells with CD45RA+ naive or CD45RO+ memory T cells purified from AB showed relatively low numbers of IL-4 and IL-10 secreting T cells in CD45RA+ AB T-cells and UCB T-cells as compared with CD45RO+ AB T cells. Numbers of IL-2 or IFN-gamma secreting cells in adult CD45RA+ T-cells were lower than in CD45RO+ T-cells but higher than in UCB T-cells. Thus diminished reactivity of UCB T-cells was not caused by a lower capacity to become activated and proliferate but may be explained by a lower extent of activation in UCB T cells, the absence of memory T cells in UCB, and differences between naive T cells from UCB and AB.