Flt 3 ligand (Flt 3-L), variously designated Flt 3/Flk 2 ligand or FL, is a hematopoietic growth factor that stimulates the proliferation of stem and CD34+ progenitor cells and has been cloned from both mouse and human genomes. Flt 3-L is a potent in vitro growth stimulator of granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and G-CSF-dependent granulocyte-macrophage committed precursors from Lin CD34+ bone marrow cells as well as other primitive B cell populations. Additionally, Flt 3-L stimulates the proliferation of hematopoietic progenitor cells isolated from mouse fetal liver or adult mouse bone marrow. Flt 3-L does not, however, affect the growth of erythroid-committed progenitors. Flt 3-L exists in two forms and is active as both a soluble and as a membrane-bound ligand. The Flt 3-L receptor, Flt 3, is a tyrosine kinase expressed on CD34+ cells that shares a high degree of homology with the SCF (stem cell factor) receptor, c-Kit and c-Fms