This paper describes the development of organic fluorescence nanoparticles. The nanoparticles have a narrow, tunable, symmetric emission spectrum and a broad, continuous excitation spectrum. The nanoparticles have high room-temperature fluorescence quantum yields and long fluorescence lifetime. They are also photochemically stable and water-soluble. They were used as fluorescence biosensor in the determination of proteins, which was proved to be a simple, rapid and specific method. In comparison with single organic fluorephores, these nanoparticles are brighter, more stable against photobleaching, and do not suffer from blinking. Under optimal conditions, the linear ranges of the calibration curves were 0.1-4.5 microg ml(-1) for human serum albumin (HSA), 0.2-3.5 microg ml(-1) for bovine serum albumin (BSA) and 0.04-0.8 microg ml(-1) for gamma globulin (gamma-IgG), respectively. The detection limits were 0.062 microg ml(-1) for HSA, 0.036 microg ml(-1) for BSA and 0.022 microg ml(-1) for gamma-IgG, respectively. However, when the content of HSA is lower than 0.8 microg ml(-1), HSA makes little contribution to the fluorescence quenching. So, the method was applied to direct selective quantification of gamma-IgG in human blood serum without separation of HSA. The results were in good agreement with these reported by the hospital, indicating that the method presented here is not only sensitive, selective and simple, but also reliable and suitable for practical applications.