The mediator originally described as a macrophage migration inhibitory factor (MIF) has been identified recently to be a novel anterior pituitary hormone and the first protein to be released from immune cells upon stimulation with glucocorticoids. Once released, MIF acts to override or counter-regulate the immunosuppressive effects of glucocorticoids on cytokine production in vitro and on LPS lethality in vivo. MIF also has been described to catalyze a tautomerization reaction, suggesting that an enzymatic activity may underlie certain of its biological properties. The three-dimensional crystal structure of human MIF has been solved at 2.6A resolution. The X-ray crystal structure reveals MIF to be a trimer of identical subunits. Each monomer contains two antiparallel alpha-helices that pack against a four-stranded beta-sheet. The monomer has an additional two beta-strands that interact with the beta-sheets of adjacent subunits to form the interface between monomers. The three beta-sheets are arranged to form a barrel containing a solvent-accessible channel that runs through the center of the protein along a molecular three-fold axis. Electrostatic potential maps reveal that the channel has a positive potential, suggesting that it binds negatively charged molecules. The elucidated structure for MIF is unique among cytokines or hormones and suggests that this glucocorticoid-induced counter-regulatory hormone has a novel mechanism of action.