FLT-3 Ligand is a member of the “potentiating” group of hematopoietic growth factors that stimulate the proliferation of hematopoietic cells. Other members of this family include stem cell factor (SCF) and colony stimulating factor 1 (CSF-1). FLT-3 Ligand has minimal proliferative activity on its own, but can synergize with a wide range of other colony stimulating factors and interleukins to stimulate proliferation. FLT-3 Ligand binds to, and transmits signals through, the receptor tyrosine kinase known as FMS-like tyrosine kinase-3 (FLT-3). FLT-3 Ligand exerts a marked stimulatory action on primitive hematopoietic progenitor cells and a unique expanding effect on putative stem cells. The distal proliferative/differentiative action of FLT-3 Ligand is largely restricted to granulomonocytic progenitors and monocytic precursors. FLT-3 Ligand does not affect the growth of erythroid committed progenitors. Expression of the FLT-3 receptor is essentially restricted to early hematopoietic progenitor cells in a quiescent status and progenitors undergoing granulomonocytic differentiation, suggesting differential expression of the FLT-3 receptor underlies the selective stimulatory effect of FLT-3 Ligand. FLT-3 Ligand also plays an important role in lymphopoiesis. 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. At least two cytokines have been identified to block FLT-3 Ligand-induced or potentiated hematopoietic activitys: TNF-alpha and TGF-beta (2-4). Human FLT-3 Ligand can stimulate the proliferation of cells expressing murine FLT-3 receptors. Recombinant Human FLT-3 Ligand ACF is a non-glycosylated protein containing 155 amino acids, with a molecular weight of 17.6 kDa.