Umbilical cord blood contains an abundance of CD34+ hematopoietic progenitor cells and has been used in transplantation as an alternative to adult bone marrow or mobilized peripheral blood. Although efficient myelomonocytic, erythroid, and B lymphoid differentiation has been shown in highly purified cord blood CD34+ mononuclear cells lacking expression of lineage-specific antigens, generation of functional T cells has not been previously documented. Exploiting two recently developed, complementary thymic stromal monolayer systems, we show here that immature hematopoietic progenitor cells (CD34+ lineage-) from human and rhesus monkey cord blood mononuclear cells undergo T lymphopoiesis in a manner that recapitulates T cell ontogeny in vivo. After 2 weeks of proliferation, cultures contained myeloid [corrected] cells and discrete populations of CD4+ CD8+(double-positive) immature T lymphocytes, followed after an additional 2 weeks by the appearance of single-positive CD4+ CD8-and CD4-CD8+ T cells that coexpressed CD3. The T lymphoid phenotype was confirmed at the transcriptional level by the presence of the lymphoid-restricted genes RAG-2 and T cell receptor. T cells generated from cord blood progenitors in these systems exhibited immunofunction as assessed by alloreactive responses in mixed lymphocyte reactions. These findings were comparable between human and rhesus progenitor cells and closely resemble previous data using adult bone marrow CD34+ cells in these models. Together, these observations demonstrate that cord blood contains abundant lymphoid progenitors that undergo T lymphopoiesis in vitro, suggesting the full multipotentiality of this stem cell source and its validity in investigating T lymphoid differentiation.