The flt3 ligand is a member of a small family of growth factors that stimulate the proliferation of hematopoietic cells; other members of this family include Steel factor (also known as mast cell growth factor, stem cell factor, and kit ligand) and colony stimulating factor 1. These proteins function by binding to and activating unique tyrosine kinase receptors. Expression of the flt3 receptor is primarily restricted among hematopoietic cells to the most primitive progenitor cells. The flt3 ligand is similar to Steel factor in that both proteins stimulate the proliferation of early progenitor or stem cells. Neither of these factors has much proliferative activity on its own, but each factor can synergize with a wide range of other colony stimulating factors and interleukins (ILs) to stimulate proliferation. One major difference between the two factors appears to be their effect on mast cells, which Steel factor stimulates but flt3 ligand does not. Although flt3 ligand and Steel factor each act on early hematopoietic cells, differences in their activities suggest that they are not redundant and both are required for normal hematopoiesis. There are a number of clinical settings in which the flt3 ligand may potentially prove quite useful.