Gold nanoparticles with a 12-nm diameter were used as probes for the determination of proteins by resonance Rayleigh-scattering techniques. In weak acidic solution, large amounts of citrate anions will self-assemble on the surface of positively charged gold nanoparticles to form supermolecular compounds with negative charges. Below the isoelectric point, proteins with positive charges such as human serum albumin (HSA), bovine serum albumin (BSA), and ovalbumin (Ova) can bind gold nanoparticles to form larger volume products (the diameter of the binding product of gold nanoparticles with HSA is 23 nm.) through electrostatic force, hydrogen bonds, and hydrophobic effects, which can result in a red shift of the maximum absorption wavelength, the remarkable enhancement of the resonance Rayleigh-scattering intensity (RRS), and the appearance of the RRS spectra. At the same time, the second-order-scattering (SOS) and frequency-doubling-scattering (FDS) intensities are also enhanced. The binding products of gold nanoparticles with different proteins have similar spectral characteristics and the maximum wavelengths are located near 303 nm for RRS, 540 nm for SOS, and 390 for FDS, respectively. The scattering enhancement (DeltaI) is directly proportional to the concentration of proteins. Among them, the RRS method has the highest sensitivity and the detection limits are 0.38 ng/ml for HSA, 0.45 ng/ml for BSA, and 0.56 ng/ml for Ova, separately. The methods have good selectivity. A new RRS method for the determination of trace proteins using a gold nanoparticle probe has been developed. Because gold nanoparticle probes do not need to be modified chemically in advance, the method is very simple and fast.