Serum transferrins are monomeric glycoproteins with a molecular mass of around 80 kDa, that transport iron to cells via receptor-mediated endocytosis. Although both serum transferrins (STfs) and ovotransferrins (OTfs) are derived from the same gene in aves, the ovotransferrins do not transport iron in vivo. Crystal structures of OTf have been solved, in contrast no three-dimensional structure of avian STf have been determined as yet. Here we report the purification, crystallization, and preliminary crystallographic studies of the hen STf both in apo- (iron free) and holo- (iron loaded) forms. The hen STf has been purified to homogeneity by hydrophobic interaction chromatography. Both the apo- and bolo-forms were crystallized by hanging drop vapor diffusion method at 277 K. The apo-crystals diffract to a resolution of 3.0 Angstrom and belong to the space group P4(3)2(1)2 with unit cell parameters a = b = 90.5 and c = 177.9 Angstrom. The holo-crystals diffract to a resolution of 2.8 Angstrom and belong to space group P2(1) with a = 72.8,.5 = 59.6, c = 88.2 Angstrom, and beta = 95.7degrees. (C) 2002 Elsevier Science (USA). All rights reserved.