Vitamin D-binding protein (DBP), a multifunctional, highly polymorphic glycoprotein responsible for the transport of vitamin D and for sequestering extracellular actin, was isolated from human serum and crystallized using vapour diffusion methods. The crystals were grown from 7.5% v/v polyethylene glycol 400 and 0.1 M acetate buffer at pH 4.6. These crystals show diffraction patterns consistent with the tetragonal space groups P4(1) and P4(3) with unit cell dimensions a = b = 135.5(4) A and c = 75.9(4) A. They diffract to 2.3 A. Using polyacrylamide gel electrophoresis it was shown that according to their electrophoretic mobility the O-glycosylated isoforms, with a terminal sialic acid residue, are absent in the crystals.