A novel recombinant interleukin-7/hepatocyte growth factor β-chain (IL-7/HGFβ) hybrid cytokine was constructed as a single chain (sc) composed of IL-7 and HGFβ connected by a flexible linker. Unlike recombinant (r) IL-7, which stimulated pro-B cells and pre-B cells only, scIL-7/HGFβ stimulated the proliferation of pre-pro-B cells, common lymphoid progenitors (CLPs), and colony-forming unit (CFU)-S₁₂ in cultures of IL-7^-/- mouse BM cells. When injected in vivo, 3- to 4-fold more splenic B-lineage cells appeared in recipients of bone marrow (BM) cells from the scIL-7/HGFβ-stimulated cultures than from rIL-7-stimulated cultures. Moreover, on a per-cell basis, scIL-7/HGFβ culture-generated cells produced 16- to 20-fold more BM and splenic B-lineage cells than did normal BM cells. Antibody blocking, receptor phosphorylation, and confocal microscopy demonstrated that scIL-7/HGFβ signals though both the IL-7 and HGF (c-Met) receptors, which form IL-7R/c-Met complexes on the surface of CLPs and pre-pro-B cells. In addition, the IL-7Rα chain, γc chain, and c-Met were coisolated from purified CLPs and pre-pro-B cells on scIL-7/HGFβ affinity gels, indicating that they are major components of the IL-7/HGFβ receptor. Hence, the present results demonstrate that the IL-7/HGFβ hybrid cytokine efficiently and selectively stimulates the most primitive B-lineage precursors in BM by inducing juxtacrine interactions between the IL-7 and c-Met receptors.