Interleukin (IL)-3, granulocyte-macrophage colony stimulating factor (GM-CSF) and IL-5 are important cytokines that control production and function of myeloid cells and dendritic cells. Over-expression of these cytokines or their receptors can lead to chronic inflammatory diseases and myeloid leukaemias. These cytokines signal through heterodimeric specific receptors consisting of cytokine-specific α chains, and a signalling subunit βc, shared by all three receptors. We have determined the 3D crystal structure of the binary GM-CSF: GM-CSF receptor α chain (GMRα) complex to 2.8 Å resolution, which reveals for the first time all three extracellular domains of the GMRα receptor chain. The structure shows a striking resemblance to the related IL-3Rα, IL-5Rα and IL-13Rα subunits; however, the differing positions of the N-terminal domains suggest that this domain may play a cytokine-specific role in cell signalling. We have used mutagenesis, ligand binding and functional studies to examine residues in GMRα and GM-CSF that define the binding interface, and identified residues involved in higher order complex assembly. Furthermore, the complete structure of the GMRα receptor chain from the binary complex was used to improve the electron density maps of the partial GMRα in the previously published ternary complex, elucidating additional GM-CSF:GMRα interactions and the position of the N-terminal domain of GMRα in the ternary complex. A structural comparison of the GM-CSF binary and ternary complexes revealed numerous major, as well as subtle, conformational changes in cytokine and both receptor chains. These studies enable us to identify differences and similarities in the way GM-CSF, IL-3 and IL-5 interact with their receptors and facilitate different types of cell signaling events