Chlorides with different paramagnetic cations such as Ni2+, Co2+ and Mn2+ were used as crystallizing agents instead of NaCl to crystallize hen egg-white lysozyme. NiCl2 was found to give two types of crystals with different morphologies: one (roof-like) is a new type of orthorhombic P2(1)2(1)2(1) crystal with lattice constants a = 79.0 Angstrom, b = 80.8 Angstrom, and c = 37.5 Angstrom; the second is an ordinary tetragonal crystal of its characteristic shape with a = b = 80 Angstrom and c = 38 Angstrom. The appearance of the roof-like shape became dominant in the presence of a magnetic field. In the case of using CoCl2 and MnCl2, ordinary tetragonal crystals were formed. A striking fact was that the a-axis of thecrystals oriented along the magnetic field when CoCl2 was used, as opposed to the usual c-axis orientation. Large and optically perfect lysozyme crystals can be obtained in a magnetic field when NiCl2 or MnCl2 is used as a crystallizing agent. These profound effects of the paramagnetic cations may be caused by the coordination of Ni2+ and Co2+ ions to a lysozyme molecule, which was found by X-ray crystallography. (C) 2003 Elsevier Science B.V. All rights reserved.