Human erythropoietin is a haematopoietic cytokine required for the differentiation and proliferation of precursor cells into red blood cells. It activates cells by binding and orientating two cell-surface erythropoietin receptors (EPORs) which trigger an intracellular phosphorylation cascade. The half-maximal response in a cellular proliferation assay is evoked at an erythropoietin concentration of 10 pM (ref. 3), 10−2 of its K d value for erythropoietin-EPOR binding site 1 (Kd 1 nM), and 10−5 of the K d for erythropoietin-EPOR binding site 2 (Kd 1 μM). Overall half-maximal binding (IC50) of cell-surface receptors is produced with ∼0.18 nM erythropoietin, indicating that only ∼6% of the receptors would be bound in the presence of 10 pM erythropoietin. Other effective erythropoietin-mimetic ligands that dimerize receptors can evoke the same cellular responses, but much less efficiently, requiring concentrations close to their K d values (∼0.1 μM). The crystal structure of erythropoietin complexed to the extracellular ligand-binding domains of the erythropoietin receptor, determined at 1.9 Å from two crystal forms, shows that erythropoietin imposes a unique 120° angular relationship and orientation that is responsible for optimal signalling through intracellular kinase pathways.