The three-dimensional structure of the macrophage migration inhibitory factor (MIF) from human lymphocytes has been determined by X-ray crystallography at 2.1 A resolution. The structure was solved by a molecular replacement technique using the coordinates of rat MIF. The molecule forms a trimer structure similar to the rat MIF. However, unlike the rat MIF whose C-terminal tail (residues 104-114) is disordered in the crystal, human MIF has a definite main-chain conformation up to the C-terminal end. These eleven residues create two more beta-strands and join to the inter-subunit beta-sheet, which contribute to forming a trimer structure. Thus, the trimer structure consists of three seven-stranded beta-sheets surrounded by six alpha-helices. Each beta-sheet is comprised of beta-strands from each of the three monomers. This architecture is almost identical to 5-carboxymethyl-2-hydroxymuconate isomerase (CHMI) and is related to the E. coli signal transducing protein PII.