Polarized Raman spectra have been obtained for tetragonal lysozyme single crystals of different relative quality. The Raman band at 507 cm-1, which corresponds to the totally symmetric stretch vibration of the gauche-gauche-gauche (ggg) disulfide bridges of the protein, has been shown to possess different polarization characteristics compared with the gauche-gauche-trans (ggt) disulfide bridge band at 528 cm-1. The relative intensities of the ggg and ggt bands in the polarized Raman spectra have been numerically estimated for a number of tetragonal lysozyme single crystals, the X-ray diffraction data of which are available from the Protein Data Bank. On the basis of comparison between the experimental and calculated polarization characteristics of the disulfide Raman lines, the following main conclusions have been drawn. The orientation of the protein molecules correlates with the average orientation of their ggg disulfide bridges. This in turn can be described by the ρggg value which reflects the average orientation of the S-S bonds with respect to the Z crystallographic axis and can be determined from polarized Raman spectra. Crystals of better quality are characterized by a better alignment of the protein molecules with respect to the Z axis, a smaller perturbation of the protein molecules in the crystal lattice and a somewhat higher interlattice water content. © 1998 International Union of Crystallography Printed in Great Britain - all rights reserved.