In recent years, crystallization techniques based on microporous hydrophobic membranes have been successfully applied to biological macromolecules. In this work, the membrane crystallizer has been used to evaluate how the presence of the polymeric surface influences the interactions between a test-protein, hen egg white lysozyme (HEWL), and two metal cations: Cu2+ and Co2+. Different techniques have been used in order to investigate the features of the obtained crystals and elucidate the membrane role during the growth Process. Experimental results show large crystal size and improved crystal quality. Unexpectedly, lysozyme-Co2+ crystals having peculiar morphologies as well as a new kind of crystal lattice (orthorhombic P2(1)2(1)2(1) form with unit cell constants a = 36.81 angstrom, b = 77.56 angstrom, c = 80.38 angstrom) have been found. Furthermore, new coordination positions of the copper cation to lysozyme, different with respect to those already described in literature, have been observed. Beside the well demonstrated effectiveness in the field of protein crystallization, these results suggest that polymeric film surfaces could be useful to control the occurring of polymorphs during crystallization and modulate the interactions between proteins and ligands like ions, additives, etc. (c) 2006 Elsevier B.V. All rights reserved.