MIF is the major secreted protein released by anterior pituitary cells and leukocytes in response to lipopolysaccharide stimulation and plays a centrai role in the toxic response to endotoxaemia and possibly septic shock. Recent investigation revealed its function to control inflammatory and immune responses. Here we present the tertiary structure of the MIF from rat liver at 2.2Å resolution. In the anaiysis, we prepared a series of selenomethionyl proteins by site-directed mutagenesis. There were three methionines in a 114 single polypeptide chain of rat MIF of which one or two methionines were replaced to alanine (positions 2 and 101). The structure was determined by the method of multiwavelength anomalous diffraction (MAD) with the use of synchrotron data from a crystai of the mutant which has one selenomethionine in a subunit. The protein forms a trimer in the crystal by inter-subunit β-sheets. Each monomer consists of two βαβ motifs aligned in quasi two-fold symmetry and makes a domain of four-stranded mixed p-sheet and two antiparallel α-helices. Unexpected similarities were detected between MIF and two kinds of isomerases.