Stem cell factor (SCF) and its receptor, c-Kit, are essential for hematopoietic, melanocyte, and germ cell development. In the hematopoietic compartment. SCF has been shown to act on cells at multiple stages of development, including effects on primitive stem cells and their more differentiated progeny. In particular. SCF can function to promote stem/progenitor cell survival, proliferation and differentiation (especially in combination with other growth factors), adhesion, activation, and migration, chemoattraction. While c-Kit is highly expressed on differentiated mast cells and SCF is a potent mast cell growth factor. c-Kit expression is down regulated during the maturation of all other hematopoietic lineages. SCF can exist in both soluble and membrane forms which may have distinct biological functions in vivo. Further understanding of this important hematopoietic regulator will provide important insights into the biology, biochemistry, and molecular biology of hematopoetic stem cells.