Flt3 ligand (FL) is a member of a small family of growth factors that stimulate the proliferation of hematopoietic cells by binding to and activating distinct tyrosine kinase receptors. Other members of this family include M-CSF and the c-kit ligand. Expression of the Flt3 receptor is primarily restricted to the most primitive hematopoietic progenitor cells, and FL stimulates the proliferation in vitro and the expansion and mobilization in vivo of stem and progenitor cells. FL as a single factor has little proliferative activity on these stem and progenitor cells, but it synergizes with a wide range of other colony-stimulating factors and interleukins to stimulate proliferation of these cells. FL is also an effective agent for mobilizing stem and progenitor cells to peripheral blood. Recent data demonstrating anti-tumor activity of FL suggest that this protein plays a major role in activating the immune system via its ability to stimulate the production of both dendritic and natural killer cells. These biologic activities of FL may potentially prove quite useful in a number of clinical settings.