Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic cytokine that exerts its effects by interaction with the GM-CSF receptor (GMR) on the surface of responsive cells. The GM-CSF receptor consists of two subunits: GMRalpha, which binds GM-CSF with low affinity, and GMRbeta, which lacks intrinsic ligand-binding capability but complexes with GMRalpha to form a high-affinity receptor (GMRalpha/beta). We conducted dynamic kinetic analyses of GM-CSF receptors to define the role of GMRbeta in the interaction of ligand and receptor. Our data show that GMRalpha/beta exhibits a higher k(on) than GMRalpha, indicating that GMRbeta facilitates ligand acquisition to the binding pocket. Heterogeneity with regard to GM-CSF dissociation from GMRalpha/beta points to the presence of loose and tight ligand-receptor complexes in high-affinity binding. Although the loose complex has a k(off) similar to GMRalpha, the lower k(off) indicates that GMRbeta inhibits GM-CSF release from the tight receptor complex. The two rates of ligand dissociation may provide for discrete mechanisms of interaction between GM-CSF and its high-affinity receptor. These results show that the beta subunit functions to stabilize ligand binding as well as to facilitate ligand acquisition.