GM-CSF is a hematopoietic growth factor that is produced by endothelial cells, monocytes, fibroblasts and T-lymphocytes following their activation by antigens or mitogens. GM-CSF stimulates the development of macrophages and neutrophils and plays a role in the proliferation and development of early erythroid megakaryocytic and eosinophilic progenitor cells. Approximately 90% of the secreted colony stimulating activities are due to GM-CSF. While non-glycosylated and glycosylated GM-CSF show similar activities in vitro, fully glycosylated GM-CSF is biologically more active in vivo compared to its non-glycosylated counterpart. Human and murine GM-CSF are species-specific and therefore, exhibit no cross-reactivity. Recombinant Human GM-CSF expressed in Human 293 cells is a 15 to 36 kDa golbular protein containing two intramolecular disulfide bonds and two potential N-linked glycosylation sites.