Granulocyte macrophage colony-stimulating factor (GM-CSF), a 22 kDa glycosylated protein, was initially characterized as a factor that can support the in vitro colony formation of granulocytemacrophage progenitors. It is also a growth factor for erythroid, megakaryocyte, and eosinophil progenitors. GM-CSF is produced by a number of different cell types (including T cells, B cells, macrophages, mast cells, endothelial cells, fibroblasts, and adipocytes) in response to cytokine or inflammatory stimuli. On mature hematopoietic cells, GM-CSF is a survival factor for and activates the effector functions of granulocytes, monocytes/macrophages, and eosinophils (1, 2). GM-CSF promotes a Th1 biased immune response, angiogenesis, allergic inflammation, and the development of autoimmunity (3 5). It shows clinical effectiveness in ameliorating chemotherapyinduced neutropenia, and GM-CSF transfected tumor cells are utilized as cancer vaccines (6, 7). Mature mouse GM-CSF shares 49% -54% amino acid sequence identity with canine, feline, human, and porcine GM-CSF and 69% with rat GM-CSF. GM-CSF exerts its biological effects through a heterodimeric receptor complex composed of GM-CSF Rα/CD116 and the signal transducing common β chain (CD131) which is also a component of the highaffinity receptors for IL-3 and IL-5 (8,9). In addition, GM-CSF binds a naturally occurring soluble form of GM-CSF Rα (10). The activity of GM-CSF is species specific between human and mouse. Mouse GM-CSF is only weakly active on rat cells, although rat GM-CSF is fully active on mouse cells (11).