To examine the role of the Notch ligand Delta-4 on hematopoietic stem cells, human CD34+CD38low cord blood cells were cocultured on S17 cells transduced with transmembrane Delta-4 (mbD4/S17) or an empty vector (C/S17). By the end of a 3-week culture, mbD4/S17 induced a 25-fold reduction in nucleated cell production, as compared to C/S17, by maintaining a higher proportion of cells in G0/G1 phase. A specific retention of a high proportion of CD34+ cells throughout the culture was observed with mbD4/S17, contrary to C/S17. Although mbD4/S17 promoted expansion of cells with the phenotype of committed lymphoid precursors (CD34+CD7+CD45RA+), these cells still retained their myeloid differentiation potential. mbD4/S17 maintained a higher LTC-IC frequency in output CD34+ cells, compared to C/S17, as in the subsets of cells having completed the same number of divisions on mbD4/S17. A Delta4-Fc protein (extracellular part of human Delta4 fused to Fc human IgG1 portion), immobilized on plastic, also reduced cell production and retained the LTC-IC potential. Transplantation of cells grown on mbD4/S17 into NOD/SCID mice showed no significant enhancement of the long-term repopulating ability. Thus, Delta4 appears to inhibit hematopoietic stem cell proliferation, in association with the maintenance of short-term lymphoid and myeloid repopulation capacity.