HLA-E is the first human class Ib major histocompatibility complex molecule to be crystallized. HLA-E is highly conserved and almost nonpolymorphic, and has recently been shown to be the first specialized ligand for natural killer cell receptors. In functional studies, HLA-E is unlike the class Ia MHC molecules in having tightly restricted peptide binding specificity. HLA-E binds a limited set of almost identical leader sequence peptides derived from class Ia molecules and presents these at the cell surface for recognition by natural killer cell receptors. We now show that the extracellular region of HLA-E forms a stable complex with beta2 microglobulin and can be refolded around synthetic peptide. Crystals of this complex formed slowly over four to six months in the presence of ammonium sulphate. The crystals diffract to 2.85 A with space group P3(1)21 and unit cell dimensions a = 182.2 A, b = 182.2 A, c = 88.4 A.