Murine mast cell development is regulated by a number of T cell- and fibroblast-derived growth factors, of which IL-3 and stem cell factor (SCF) appear to be the most important. To determine the relative effects of these two growth factors on different stages of developing mast cells, we isolated highly purified populations of c-kit+ mast cells and their progenitors and examined their ability to respond to SCF and/or IL-3. We show that purified c-kit+ cells isolated from bone marrow, mesenteric lymph nodes of Nippostrongylus brasiliensis-infected mice, and the peritoneal cavity do not form mast cell colonies in response to SCF as a single agent. Although unpurified populations of lymph nodes of Nippostrongylus brasiliensis-infected mice or peritoneal cells were able to form mast cell colonies in response to SCF alone, this probably reflects paracrine factors produced by accessory cells. IL-3 alone did not promote mast cell colony formation from either population. Within bone marrow, we detected both multipotential and unipotential mast cell progenitors. Interestingly, IL-3 alone promoted only the development of multipotential progenitors, while SCF plus IL-3 promoted both multipotential and unipotential mast cell progenitors. Together, these results indicate that the developmental/proliferative effects observed with a particular mast cell population is determined in part by the stage of maturation of mast cells at the time they are exposed to SCF and IL-3, and suggest that SCF primarily influences mast cell development only in the context of other cytokines.