Abstract Notch and its ligands regulate multiple cell fate decisions. However, several questions on the timing, durability, and reversibility of Notch signaling effects on human hematopoietic precursors are still unresolved. Here, we used recombinant Delta ligands to deliver temporally and dose-controlled signals to human immature cord blood CD34+CD38low cells at clonal cell levels. Notch activation increased the frequency of multipotent progenitors, skewed the T and natural killer (NK) cell potential of CD34+CD38low clones in a dose- and ligand-dependent manner, and inhibited the differentiation of B cell clones. Low doses of ligands were sufficient for significantly increasing the frequency of NK cell precursors, whereas higher doses were required for increasing the frequency of T-cell clones. Interestingly, we demonstrate that temporary Notch activation prevents the subsequent differentiation of CD34+CD38low cells beyond a pro-B CD79a+CD19− stage characterized as a common lymphoid progenitor (CLP). Moreover, the lymphoid potential of this pro-B/CLP was skewed toward NK cell potential while the B cell precursor frequency was dramatically reduced. These results indicate critical timing and quantitative aspects of Notch/Delta interactions, imprinting the potential of CD34+CD38low hematopoietic progenitors. These results may have implications both in physiology and for cell manipulation because they demonstrate a tight regulation of the fate of human progenitors by Notch signaling. STEM CELLS 2009;27:1676–1685 Disclosure of potential conflicts of interest is found at the end of this article.