The fluorescence of 9, 10-dioxa-syn-3, 4, 6, 7-tetramethylbimane (bimane) was found to be quenched in the presence of guanosine 5'-monophosphate. By using this phenomenon, the bimane system was used for the fluorophor of substrates for phosphodiesterase I.Bimanes were coupled to 5'-guanylic acid and the resulting compounds were shown to be portent fluorogenic substates for the assay of phosphodiesterase I.