The crystal structure of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) has been determined at 2·8 Å resolution using multiple isomorphous replacement techniques. There are two molecules in the crystallographic asymmetric unit, which are related by an approximate non-crystallographic 2-fold axis. The overall structure is highly compact and globular with a predominantly hydrophobic core. The main structural feature of rhGM-CSF is a four α-helix bundle, which represents approximately 42% of the structure. The helices are arranged in a left-handed antiparallel bundle with two overhand connections. Within the connections is a two-stranded antiparallel β-sheet. The tertiary structure of rhGM-CSF has a topology similar to that of porcine growth factor and interferon-β. Most of the proposed critical regions for receptor binding are located on a continuous surface at one end of the molecule that includes the C terminus. © 1992.