GM-CSF is a 22 kDa monomeric hematopoietic cytokine that was initially characterized as a growth factor that supported the in vitro colony formation of granulocyte-macrophage progenitors. It is produced by a number of different cell types, including activated T cells, B cells, macrophages, mast cells, endothelial cells and fibroblasts, in response to cytokine or immune and inflammatory stimuli. GM-CSF is species-specific and human GM-CSF has no biological effect on mouse cells.