This chapter discusses the murine granulocyte-macrophage colony-stimulating factor. The differentiation and proliferation of hemopoietic progenitor cells is highlighted, that it is dependent on a set of specific glycoproteins. One of these glycoproteins is granulocyte-macrophage colony-stimulating factor (GM-CSF), which stimulates the proliferation in vitro of specific murine hemopoietic progenitor cells to form colonies containing mature neutrophilic granulocytes and macrophages. GM-CSF occurs in animal sera and the culture medium from a variety of tissues. GM-CSF is purified from mouse lung conditioned medium (MLCM) and has an apparent molecular weight of 23,000. cDNA clones corresponding to the mRNA from the murine GM-CSF gene are obtained and fully sequenced. GM-CSF is distinct from several other hemopoietic growth factors such as multi-CSF (also called IL-3) which also stimulate the production of mature erythroid cells from erythroid precursors and the continuous proliferation of mast cell lines. The chapter further explains purification of GM-CSF.