Granulocyte Macrophage Colony Stimulating Factor (GM-CSF) is essential for proliferation and development of granulocyte and monocyte/ macrophage progenitors. It functions as a growth factor for erythroid and megakaryocyte precursor cells. It is a potent chemoattractant for neutrophils and eosinophils and enhance effector function of neutrophils and macrophages. It has been shown to promote dendritic cell differentiation and proliferation in vitro. It has many synergistic interactions with other growth factors. The human and murine molecules are species-specific and exhibit no cross-species reactivity.