The polystyrene-acrylic acid (PS-AA) nanoparticles have been prepared by ultrasonic polymerization, characterized by FT-IR and TEM. It is the first report on the determination of proteins with macromolecules nanoparticles of PS-AA by resonance light-scattering (RLS). At pH 6.9, the RLS of macromolecules nanoparticles of PS-AA can be enhanced by proteins. Based on this, a novel quantitative assay of proteins at the nanogram levels has been proposed. At pH 6.9, the RLS signals of PS-AA were greatly enhanced by proteins in the region of 250-700 nm characterized by the peak at 342 nm. Under optimal conditions, the linear ranges of the calibration curves were 0.02-11.0 microgml-1, 0.04-10.0 microgml-1 and 0.03-10.0 microgml-1 for gamma-globulin (gamma-IgG), bovine serum albumin (BSA) and human serum albumin (HSA), respectively. The detection limits were 16.0 ngml-1, 19.0 ngml-1, and 15.0 ngml-1 for gamma-IgG, BSA and HSA, respectively. The method has been applied to the analysis of total proteins in human serum samples collected from the hospital and the results were in good agreement with those reported by the hospital, which indicates that the method presented here is not only sensitive, simple, but also reliable and suitable for practical application.