The components of the gyration tensor of the enzyme lysozyme were measured by using the HAUP method: g11 = -0.90 x 10(-5) and g33 = 1.05 x 10(-5) at 303.4 K and a wavelength of 4880 A. The optical rotatory powers along the a and c axes in the same conditions were calculated: rho a = -21.3 and rho c = -24.8 degrees cm-1. The optically active property of lysozyme is strange in that, although it contains a considerable quantity of alpha-helices (about 30%), the rotatory powers are unexpectedly small in magnitude, one order of magnitude less than those of quartz and with very large anisotropy. A conceptual consideration of this phenomenon is given. In order to assess the difference between the structures in both crystalline and solution states, the chirality index r was calculated to be 0.16. This value indicates that the structural change of lysozyme from the solution into the crystalline state is expressed by an increase of 19% in optical activity. From the NMR results [Smith et al. (1993), J. Mol. Biol. 229, 930-944], it is anticipated that the r value reflects the increased constraint in atomic motion in the side chains of exposed amino acid residues in the crystalline state.