Signaling through the IL-7 receptor (IL-7R) is necessary for the development of the earliest B-and T-lineage cells. IL-7R is ®rst expressed on common lymphoid progenitor cells and is not detected on primitive common myeloid progenitors. In this study, we show that enforced expression of IL-7R on multipotential stem cells does not in¯uence lymphoid versus myeloid cell fate. T cell development was compatible with sustained IL-7R expression; however, we observed a near complete block in B cell development at the onset of B-lineage commitment. Unlike preproB cells from control animals, developmentallyarrested IL-7R + B220 + CD19 ± NK1.1 ± Ly-6C ± cells failed to express EBF and Pax5. These results suggest that transient downregulation of IL-7R signaling is a necessary event for induction of EBF and Pax5 expression and B-lymphocyte commitment.