FLT-3 ligand promotes long- term expansion and differentiation of human pro-B cells in the presence of IL-7 or in combination of IL-7 and IL-3. The human flt-3 ligand also stimulates the proliferation of cells expressing murine flt-3 receptors. In combination with SCF and IL3, flt-3 ligand can cause expansion of cells with the marker spectrum CD34(+) CD38(-). Alone, flt- 3 ligand supports the survival of precursors in the lineage of blood-forming cells such as CFU-GM, CFU- GEMM, and the very primitive high proliferative potential colony-forming cells, HPP-CFC. flt-3 ligand only has marginal effects on progenitors for erythroid cells and megakaryocytes.