Cation-exchange high performance liquid chromatography at pH 6, developed originally to purify human lysozyme, was applied to hen egg-white lysozyme. We could remove at least three kinds of impurities from the commercial product. The impurities were considered to be modified lysozyme molecules, mostly based on N-terminal amino acid analyses. Atomic force microscopic observation was made on the crystals both from the purified and non-purified solutions. The (110) faces of the orthorhombic crystals grown at 40degreesC from the purified solution contained linear steps, while most of the linear edges became round and rugged on the crystals from non-purified solutions. A similar change in step morphology is known to occur on insulin crystals when two amino acids were mutated from the wild type. On the (010) face, elongated, round steps became rugged when crystals grew from non-purified solutions. (C) 2002 Elsevier Science B.V. All rights reserved.