Ligands for flt3 receptors capable of transducing self-renewal signals to regulate the growth, proliferation or differentiation of progenitor cells and stem cells are disclosed. The invention is directed to Flt3-ligand as an isolated protein, the DNA encoding the Flt3-ligand, host cells transfected with cDNAs encoding Flt3-ligand, compositions comprising Flt3-ligand and methods of using Flt3-ligand in hematopoietic cell transplantation.