Granulocyte-macrophage colony stimulating factor (GM-CSF) is a growth and differentiation factor for cells in the granulocyte, macrophage and eosinophil lineage. GM-CSF stimulates colony formation from pluripotential progenitor cells at extremely low concentrations and is an essential survival and proliferative factor for hematopoietic progenitor cells in all divisions up to maturity. It also stimulates growth in some epithelial cells and osteoclasts. GM-CSF is produced by a variety of cell types (monocytes, endothelial cells, T-cells, fibroblasts, mitogen-stimulated B-cells, and LPS-stimulated macrophages). GM-CSF is secreted as a single chain glycoprotein containing 128 amino acids for human with a conserved disulfide bond. Human and murine GM-CSF share approx. 54% sequence homology and do not cross-react in bioactivity.