Granulocyte-macrophage colony-stimulating factor (GM-CSF) was originally described as a potent stimulus of the growth and differentiation of granulocyte and macrophage precursor cells in vitro. It is an approximately 23 к Da glycoprotein with a four a helical bundle structure that binds to a heterodimeric receptor composed of subunits belonging to the type 1 cytokine receptor family. Subsequent studies revealed that it also stimulated the mature end cells, as well as antigen-presenting dendritic cells, to increase their functional capacity in combating infections. Somewhat surprisingly, genetic ablation experiments in mice indicated that GM-CSF was not absolutely required for steady state hematopoiesis. but was essential for maintaining the functional activity of some macrophage populations such as those involved in clearing surfactant in the lung and in responding to certain kinds of infection or immune responses. GM-CSF is in current clinical use for enhancing hematopoietic recovery following cancer chemotherapy with or without bone marrow or peripheral blood stem cell transplants.