In order to understand the biological properties of the immune systems of waterfowl and to establish a system for structural studies of duck class I major histocompatibility complex (DuMHC I), a complex of DuMHC I with duck beta(2)-microglobulin (Dubeta(2)m) and the peptide AEIEDLIF (AF8) derived from H5N1 NP residues 251-258 was assembled. The complex was crystallized; the crystals belonged to space group C222(1), with unit-cell parameters a = 54.7, b = 72.4, c = 102.2 A, and diffracted to 2.3 A resolution. Matthews coefficient calculation and initial structure determination by molecular replacement showed that the crystals did not contain the whole DuMHC I complex, but instead contained the DuMHC I alpha3 domain and a Dubeta2m molecule (DuMHC I alpha3+beta2m). Another complex of DuMHC I with the peptide IDWFDGKE derived from a chicken fusion protein also generated the same results. The stable structure of DuMHC I alpha3+beta2m may reflect some unique characteristics of DuMHC I and pave the way for novel MHC structure-related studies in the future.