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Among a novel series of synthetic thrombin-inhibitors belonging to arginine derivatives found by S. Okamoto et al. , No. 205, dansyl-arginine-piperidineamide, was reported by Mann and his associates to show the enhancement of fluorescence intensity when bound to thrombin. However, No. 205 was reported by S. Okamoto et al. to inhibit pseudocholinesterase with very low Ki also, while No. 805, (4R- methyl- [ methyl-tetrahydro-quinolinesulfonyl-arginyl- ] 2R- piperidinecarboxylic acid), showed practically no inhibitory action on pseudocholinesterase remaining very potent inhibitory action to thrombin with a Ki of 0.019 μM. Therefore, comparative studies of fluorescence variation with No. 205 and No. 805 were made using thrombin, pseudocholinesterase and truecholinesterase as the target enzymes and the following results were obtained. 1) The enhancement of fluorescence intensity of No. 205 with thrombin was corn- firmed but the same order enhancement of fluorescence intensity of No. 205 was also observed with pseudocholinesterase, but not with truecholinesterase (to this enzyme Ki of No. 205 was large), indicating that the enhancement is concomitant with the inhibitory binding of No. 205 with the enzymes. 2) The enhancement of fluorescence intensity of No. 205 with thrombin was clearly reversed by the addition of No. 805, indicating the replacement of No. 205 bound to the thrombin by No. 805. This supports the possibility of the fluorometric differentiation of thrombin from pseudocholinesterase, which allows us to discuss the histo- chemical application of synthetic thrombin inhibitors to demonstrate the local thrombin.