Granulocyte colony-stimulating factor (GCSF) is the principal growth factor regulating the maturation, proliferation and differentiation of the precursor cells of neutrophilic granulocytes' and is used to treat neutropenia(2), GCSF is a member of the long-chain subtype of the class 1 cytokine superfamily, which includes growth hormone, erythropoietin, interleukin 6 and oncostatin M (ref. 3), Here we have determined the crystal structure of GCSF complexed to the BN-BC domains, the principal ligand-binding region of the GCSF receptor (GCSFR). The two receptor domains form a complex in a 2:2 ratio with the ligand, with a noncrystallographic pseudo-twofold axis through primarily the interdomain region and secondarily the BC domain. This structural view of a gp130-type receptor-ligand complex presents a new molecular basis for cytokine-receptor recognition.