To study the effects of impure proteins, i.e. proteins with heterogeneous features that are minor constituents within "purified" protein solutions, on the crystallization of target proteins, and in particular the effects of such impurities on intermolecular contacts, we modified only the e-amino group of the N-terminal lysine of hen egg-white lysozyme with a fluorescent reagent, tetramethylrhodamine-5-isothiocyanate (5-TRITC), which has a high detection sensitivity. We then investigated the effects of the fluorescent-labeled lysozyme (Flysozyme) on the nucleation and growth of tetragonal and monoclinic lysozyme crystals. In the tetragonal and monoclinic crystals, the fluorescent labels were located at molecular surfaces of lysozyme inside and outside of the intermolecular contact areas, respectively. We found that addition of a low concentration (0.02 mg/ml) of F-lysozyme significantly suppressed nucleation of the tetragonal crystals but had no effect on that of the monoclinic crystals. In contrast, addition of a higher concentration (0.05 mg/ml) of F-lysozyme significantly promoted the heterogeneous nucleation of both polymorphs. In addition, the decrease in the growth rate of the tetragonal crystal by Flysozyme was much more significant than that of the monoclinic one, although the effective distribution coefficients of F-lysozyme for the tetragonal and monoclinic crystals were similar (2.9 and 3.2, respectively). These results clearly indicate that inhibiting the formation of specific intermolecular contacts plays a crucial role in the effects of impure proteins. (c) 2006 Elsevier B.V. All rights reserved.