US nanocrystals with an average width of about 6 nm and narrow size distribution were synthesized in aqueous solution of pepsin at room temperature. The obtained US nanocrystals were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). The quantum-confined effect of the US nanocrystals is confirmed by the ultraviolet-visible (UV-vis) and photoluminescence (PL) spectrum. The results indicate that the pepsin not only induce the nucleation, but inhibit the further growth of US nanocrystals. The effect of Cd2+ on pepsin and the change of pepsin conformation were studied through Fourier transform infrared (FTIR) spectroscopy. The possible mechanism of US nanocrystals growth in the pepsin solution was also discussed. (c) 2005 Elsevier B.V All rights reserved.