A novel fluorescence probe based on the quenching of fluorescence of bovine serum albumin (BSA) capped ZnS nanoparticles (NPs) has been developed for the determination of gatifloxacin in bulk and pharmaceutical dosage form. The functionalized ZnS NPs were successfully synthesized in aqueous medium, with the biomolecule BSA as the capping ligand. Due to a specific interaction, the fluorescence intensity of ZnS NPs reduces obviously in the presence of gatifloxacin. Under the optimal conditions, the relative fluorescence intensity of ZnS NPs is linearly proportional to gatifloxacin concentration from 2.0 x 10(-7) M to 1.0 x 10(-4) M with a detection limit of 6.5 x 10(-8) M. The proposed method is simple, rapid, practical and can be successfully applied to assay gatifloxacin capsules. (C) 2012 Elsevier B.V. All rights reserved.