In this paper, we confirm data reported by the group of Zúñiga-Pflücker that human cord blood CD34(+)38(-)Lin- progenitor cells when co-cultured with the murine stromal cell line OP9-DL engineered to express the Notch ligand delta-like-1 mature into T lymphocytes with a phenotypic progression as the one seen in thymus. We show that this is also the case for human T cells starting from CD34(+) adolescent bone marrow cells. These findings offer the theoretical possibility to generate ex vivo human T cells and administer them in vivo in patients to overcome their immune deficient window period after transplantation. However, the practical and theoretical problems that this new technology has to overcome before this technique can be applied in clinic are still enormous and discussed.