We examined the effects of applying ultrasonic power to the crystallization of hen egg white lysozyme (HEWL). HEWL was crystallized by a batch method. The samples were irradiated using ultrasonic power (100 kHz and 100 W) for 10 s and 1 min at 0. and 30 min after the crystallization experiments began. Microscopic observations were performed at regular intervals to confirm nucleation of the HEWL crystals. The number of nucleated crystals and the rate of nucleation were increased by ultrasonic irradiation for 10s immediately after the preparation of crystallization, whereas ultrasonic irradiation for I min did not have any additional effect. In contrast, irradiation 30 min after the preparation of crystallization delayed the time required for nucleation. These results suggest that long-term irradiation and irradiation in solution with clusters of protein molecules that include crystal nuclei damage the clusters, while brief ultrasonic irradiation in protein solution without clusters promotes nucleation and generates crystal nuclei. (c) 2006 Elsevier B.V. All rights reserved.