GM-CSF is a small glycoprotein growth factor which stimulates the production and function of neutrophils, eosinophils and monocytes. It can be produced by a wide variety of tissue types. The biological effects of GM-CSF are mediated through its binding to cell surface receptors, which appear to be widely expressed by thermatopoietic cells and also by some non-thermatopoietic cells, such as endothelial cells.