The recombinant macrophage migration inhibitory factor (MIF) of a rat liver was crystallized using the hanging-drop vapor diffusion method. We also crystallized a selenomethionyl rat MIF under similar conditions for X-ray structure analysis using the multiwavelength anomalous diffraction method. Furthermore, two kinds of selenomethionyl rat MIF mutants, in which one or two of three methionine residues were replaced by alanine [(Met101Ala), (Met2Ala and Met101Ala)], were constructed and crystallized. These crystals belong to two different forms of the hexagonal space group P6(3). The first form has cell dimensions of a = b = 95.7 Angstrom and c = 87.9 Angstrom and contains two dimers in an asymmetric unit, while the second form, whose cell dimensions are a = b = 61.8 Angstrom and c = 53.4 Angstrom, contains only one subunit in an asymmetric unit. Preliminary X-ray examination shows that these crystals are suitable for high-resolution structure analysis. (C) 1995 Academic Press, Inc.