Interleukin-3, also known as mast cell growth factor, P-cell stimulating factor, burst promoting activity, multicolony stimulating factor, t h y-1 inducing factor and WEHI-3 growth factor, is a pleiotropic factor produced primarily by activated T cells that can stimulate the proliferation and differentiation of pluripotent hematopoietic stem cells as well as various lineage committed progenitors. In addition, IL-3 also affects the functional activity of mature mast cells, basophils, eosinophils and macrophages. In addition to activated T cells, other cell types such as human thymic epithelial cells, activated murine mast cells, murine keratinocytes and neurons/astrocytes can also produce IL-3. Mature human IL-3 share only 29% aa sequence identity with murine IL-3. IL-3 activity is highly speciesspecific and human IL-3 does not show activity on murine cells. IL-3 is involved in a variety of cell activities such as cell growth, differentiation and apoptosis. This cytokine has been shown to also possess neurotrophic activity, and it may be associated with neurologic disorders.