The structure of murine leukemia inhibitory factor (LIF) has been determined by X-ray crystallography at 2.0 A resolution. The main chain fold conforms to the four alpha-helix bundle topology previously observed for several members of the hematopoietic cytokine family. Of these, LIF shows closest structural homology to granulocyte colony-stimulating factor and growth hormone (GH). Sequence alignments for the functionally related molecules oncostatin M and ciliary neurotrophic factor, when mapped to the LIF structure, indicate regions of conserved surface character. Analysis of the biological function and receptor specificity of a series of human-mouse LIF chimeras implicate two regions of receptor interaction that are located in the fourth helix and the preceding loop. A model for receptor binding based on the structure of the GH ligand-receptor complex requires additional, novel features to account for these data.