Nanometer-sized L-cysteine-capped ZnS particles have been synthesized and used as a fluorescence probe to investigate the effect of proteins on fluorescent intensity. With Delta lambda=190 nm, maximum and constant synchronous fluorescence enhancement was produced at 267 nm and pH 5.12 in the presence of proteins. A highly sensitive synchronous fluorescence method for the rapid determination of proteins has been developed. Under optimum conditions, calibration graphs are linear over the range 0.03-8.0 microg mL(-1) for bovine serum albumin (BSA), 0.01-6.0 microg mL(-1) for human serum albumin (HSA), 0.05-8.0 microg mL(-1) for gamma-globulin (gamma-G), and 0.04-4.0 microg mL(-1) for ovalbumin, respectively. The relative standard deviations of seven replicate measurements were 1.75% for 1.0 microg mL(-1) BSA, 1.90% for 1.0 microg mL(-1) HSA, 1.65% for 1.0 microg mL(-1) gamma-G, and 2.32% for 1.0 microg mL(-1) ovalbumin.