Interleukin-3 is a hemopoietic growth factor involved in the survival, proliferation, and differentiation of multipotent hemopoietic cells. IL-3 is the most potent growth factor for basophils, followed by granulocyte-macrophage colony-stimulating factor and IL-5. These cytokines also act on mature basophils through specific receptors, thereby mediating adhesion, migration, and releasability. IL-3 is highly expressed by mast cells, and large and rapidly released amounts of autocrine IL-3 production are responsible for mast cell survival by IgE in the absence of antigen. IL-3 has also been implicated in the pathogenesis of several chronic inflammatory diseases, including asthma, atherosclerosis, and neurodegenerative disorders, such as multiple sclerosis.? IL-3 stimulates colony formation of megakaryocytes, neutrophils, and macrophages from bone marrow cultures. IL-3 plays a vital role in stimulating basophils and mast cell responses to parasite infections, and since basophils play an important role in Th2 immune responses, IL-3 is a critical regulator of allergic inflammation. IL-3 is expressed in the major embryonic vessels and regulates the survival and proliferation of hematopoietic stem cells in the early stages of embryonic development.