Mast cells and basophils express certain remarkable similarities in mediator content, histochemical characteristics, and function. Yet a large body of evidence now indicates that the mast cell and basophil lineages are distinct. Stem cell factor, the ligand for the receptor encoded by c-kit, is a major growth factor for mast cells in both rodent and primate species, and can modulate mast cell secretory function. Moreover, abnormalities affecting the stem cell factor receptor or stem cell factor might contribute to some cases of mastocytosis or mast cell neoplasms. By contrast, basophils can develop independently of stem cell factor, and are not as sensitive as mast cells to the effects of stem cell factor on mediator secretion. In addition, the cytokine interleukin-3 greatly augments the production of human basophils, but has little or no growth promoting activity for human mast cells.