A selective detection system based on on-line post-column fluorescence derivatization is described for the analysis of arginine-containing peptides by high-performance liquid chromatography. The peptides are automatically converted into fluorescent derivatives with benzoin, a fluorogenic reagent for guanidino compounds, after separation on a reversed-phase column (TSKgel ODS-120T) and detection in an ultraviolet absorption detector. The system permits fluorescence detection at 435 nm emission with irradiation at 325 nm for arginine-containing peptides in as little as picomole amounts. The chromatogram obtained with fluorescence detection only shows peaks corresponding to arginine-containing peptides. The facile detection of arginine-containing fragments in the tryptic digest of beta-melanocyte stimulating hormone as a model compound could be achieved by comparison with a chromatogram obtained with ultraviolet absorption detection at 215 nm.