Sensitive, reactive, and hydrophilic fluorogenic reagents for thiols with the benzofurazan skeleton, 4-(N-acetylaminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (AcABD-F) and 4-(N-trichloroacetylaminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole (TCAcABD-F) have been developed. These reagents reacted with thiols within 10 min at 60 degrees C. AcABD-F and TCAcABD-F themselves do not fluoresce but are strongly fluorescent after the reaction with thiol compounds. The generated derivatives were highly water-soluble, since they dissociated a proton and ionized in the neutral pH region. The derivatives with four biologically important thiol compounds were separated on a reversed-phase HPLC column and detected fluorometrically at 504 nm with excitation at 388 nm. The detection limit attained for homocysteine with AcABD-F was 25 fmol on column (11 nM) (signal-to-noise ratio = 3), and that for glutathione with TCAcABD-F was 45 fmol on column (20 nM).