Crystals of the 14-kDa group 5 allergen from Dermatophagoides pteronyssinus (Der p 5) have been obtained at low pH and diffract to 3-Angstrom resolution using a conventional x-ray source. The crystals belong to tetragonal space group P4(1)2(1)2 or P4(3)2(1)2, with unit cell parameters a = b = 114 Angstrom and c = 234 Angstrom. A self-rotation search revealed a 432 point symmetry and thus suggested 96 molecules in one unit cell, hence 12 monomers in each asymmetric unit.