Orthorhombic crystals of hen egg-white lysozyme (HEWL) were grown under a homogeneous magnetic field of 10 T (B = 10 T). In the magnetic field, crystals were oriented such that their crystallographic c-axes were parallel to the magnetic field, and gave narrower average rocking widths than those grown under 0 T. Crystal quality was evaluated using the full-width at half-maximum (FWHM) of the rocking curve, and the effects of a magnetic field were examined by comparing FWHM of seven crystals grown at 10 T with an equal number grown at 0 T. FWHM values in (10 0 0), (0 8 0), and (0 0 6) reflections decreased by 25%, 38%, and 32%, respectively, and those of general reflections decreased by 7-45 % in crystals grown under high magnetic field. These results show that a magnetic field of 10 T improved the crystal perfection of the orthorhombic lysozyme crystals. At the same time, the maximum resolution limit of X-ray diffraction increased from 1.33 Angstrom for 0 T to 1.13 Angstrom for 10 T. These facts suggest that the application of magnetic field for crystallization might generally have a striking effect in enhancing the diffraction power of protein crystals.