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Because a relatively low incidence and severity of graft-versus-host disease after umbilical cord blood (UCB) transplantation is observed, we investigated whether T cells from UCB or adult blood (AB) were differentially activated by antigen-presenting cells with or without human leukocyte antigen (HLA)-DR expression. T cells from UCB or AB, or CD45RA(+) naive T cells and CD45RO(+) memory T cells separated from AB, were stimulated with the HLA-DR(+) or HLA-DR(-) cell line AML193. On days 1-3 after stimulation, numbers of interleukin (IL)-2, IL-4, IL-10 or interferon gamma (IFN-gamma)-secreting cells were determined by enzyme-linked immunospot analysis. No IL-4 or IL-10 was produced. AML193-DR(+) cells induced IL-2 and IFN-gamma secretion with slower kinetics and lower levels in UCB T cells than in AB T cells. AML193-DR(+) cells induced comparable IL-2 but higher IFN-gamma secretion in CD45RA(+) T cells from AB than in UCB T cells. AML193-DR(-) cells did not induce IL-2- or IFN-gamma secretion in UCB T cells, but stimulated both CD45RA(+) and CD45RO(+) T cells from AB to secrete IL-2 and IFN-gamma. Thus, not only the absence of memory T cells but also the inability to respond to HLA-DR-negative antigen-presenting cells and the slower kinetics and level of activation found for naive T cells from UCB as compared with AB may partly explain the reduced antirecipient reactivity after UCB transplantation.