In this work, the effects of an internal electric field (a direct current) on the crystallization of hen egg-white lysozyme were investigated. The crystallization was carried out in solution and gel placed in a crystallization cell. This crystallization growth cell has two parallel Pt electrodes (0.2 mm diameter and 15 mm length), and those electrodes were in contact with crystallization media. A direct current of 2.0 muA was applied (1.9 x 10(-3)V) for 8 h in solution and 12 h in gel media. During the crystallization, no generation of gas was observed. Lysozyme crystals appeared only around the cathode (negatively charged electrode), while amorphous precipitation was observed around the anode (positively charged electrode). By applying the direct current, the number of the deposited crystals was significantly decreased, and consequently the size of the crystals was increased. The direct current also decreased the induction time of nucleation. No significant change in pH was observed during the crystallization. The deposition of the crystals only around the cathode and the decrease in the induction time can be explained by an opposite polarity of the lysozyme molecules and the electrodes and by interactions between the lysozyme molecules and the Cl- in the vicinity of the electrodes. Electric potential generated by an electric field would be also responsible for the decrease in the number of the deposited crystals. (C) 2004 Elsevier B.V. All rights reserved.