Highly fluorescent gold clusters have been synthesized in basic aqueous solution by using lysozyme as reducing and stabilizing agents. The lysozyme-stabilized gold fluorescent clusters (LsGFC) have an average size of 1 nm and emission approximately 657 nm. The fluorescence could be specifically quenched by Hg(2+), so the LsGFC can be used as a sensor for sensitive and selective Hg(2+) detection with a detection limit of 10 nM.