A novel composite nanoparticle has been prepared by an in situ polymerization method and applied as a protein fluorescence probe. The nano-CdS has been prepared, then the polymerization of acrylic acid (AA) was carried out by initiator potassium persulfate (KPS) under ultrasonic irradiation. The surface of the composite nanoparticles was covered with abundant carboxylic groups (--COOH). The nanoparticles are water-soluble, stable, and biocompatible. The synchronous fluorescence intensity of the composite nanoparticles is significantly increased in the presence of trace protein at pH 6.90. Based on this, a new synchronous fluorescence scan (SFS) analysis was developed for the determination of proteins including BSA, HSA, and human gamma-IgG. When Delta lambda = 280 nm, maximum synchronous fluorescence is produced at 290 nm. Under the optimum conditions, the response is linearly proportional to the concentration of proteins. The linear range is 0.1-10 microg ml(-1) for HSA, 0.09-8.0 microg ml(-1) for BSA, and 0.08-15 microg ml(-1) for human gamma-IgG, respectively. The method has been applied to the determination of the total protein in human serum samples collected from the hospital and the results are satisfactory.