Somatic mutations of FLT3 involving internal tandem duplication (ITD) of the juxtamembrane domain or point mutations in the tyrosine kinase domain (TKD) appear to activate FLT3 in a FLT3 ligand (FL)-independent manner. To determine whether or not FLT3 mutants respond to FL for their activation, a FL-deficient (FL(-/-)) murine embryo fibroblast (MEF) cell line was established. Expression of FLT3/ITD and FLT3/TKD mutations in FL(-/-) MEF cells resulted in low levels of constitutive phosphorylation of FLT3. However, a more than fourfold increase of FLT3 autophosphorylation was induced by exogenous FL. Rescue of endogenous FL expression in FL(-/-) MEF cells expressing FLT3 mutants led to more than a threefold increase of FLT3 phosphorylation. FL addition led to further activation of the FLT3 receptors and enhanced survival and/or decreased apoptosis in leukemia-derived cell lines and primary leukemic cells expressing FLT3 mutations. Functional studies revealed that exogenous FL promoted the colony-forming and recloning abilities of FLT3 mutant transduced primary bone marrow cells derived from FL(-/-) mice. Endogenous FL contributes in vivo to functional signaling through FLT3 as noted by the decreased survival of FL(+/+)ITD(+/+) mice compared with FL(-/-)ITD(+/+) mice. These data suggest that FL leads to further activation of FLT3 mutants and is especially important in light of recent findings of elevated FL levels in acute myeloid leukemia patients in response to chemotherapy.