It is well established that mast cells, which are found in the tissues in the proximity of small blood vessels and post-capillary venules, play a key role in the early phase of IgE-mediated allergic reactions. A greatly expanded understanding of the biology of IL-3 has emerged since the early 1980s. IL-3 is a specific factor that stimulates the growth of hematopoietic stem and progenitor cells of a variety of lineages and can promote the proliferation of certain classes of lymphocytes distinct from those that are dependent on IL-2. IL-3 has been identified among the most important cytokines for regulation of mast cell growth and differentiation, migration and effector function activities of many hematopoietic cells. IL-3 termed multi colony-stimulating-factor (multi-CSF) or mast cell growth factor (MCGF) is a haematopoietic growth factor which stimulates the formation of colonies for erythroid, megakaryocytic, granulocytic and monocytic lineages. It is predominantly produced by activated T cells, natural killer (NK) cells and mast cells and supports the growth-promoting effects of SCF on mast cell precursors. IL-3 causes severe hypersensivity reactions and plays a pivotal role in exacerbating the inflammatory response in vivo. Here we report the interrelationship between IL-3 and mast cells.