A newly synthesized Edman-type fluorogenic reagent, 4-(2-cyanoisoindolyl)phenylisothiocyanate, reacted with 20 tested amino acids in a basic medium to produce the corresponding thiocarbamoyl derivatives. The derivatives were converted in an acidic medium to the thiohydantoin derivatives which were separated by reversed-phase chromatography and detected by fluorimetry. The on-column detection limits for amino acids ( S/N = 3) were on the subpicomole level. Dipeptides, LeuAla and AlaGly also reacted with the reagent to produce the thiocarbamoyl derivatives which were cleft in an acidic medium at the peptide bonds to produce the thiohydantoin derivatives of the N-terminal amino acids.