The function of the G-CSF receptor has been of considerable interest, particularly because of the clinical usefulness of G-CSF. The first step in receptor activation, which is the interaction of G-CSF with its receptor, has been studied by mapping the binding sites of neutralizing antibodies, by studying the complexes formed between G-CSF and various receptor fragments in solution and by mutagenesis of the receptor and ligand. In addition, the structure of G-CSF has been determined. Part of the ligand-binding domain of the receptor in complex with G-CSF has been crystallized and its structure described. Consideration of all these studies has allowed us to make a model of the complete ligand-binding domain in complex with G-CSF that accounts for the published data. The complex has a 2:2 stoichiometry, with two binding sites on both the ligand and receptor that are equivalent to site II and site III of the IL-6 receptor complex. This model was based on the published structure of gp130 in complex with viral IL-6, which we believe to be very similar.