A synthetic esteroprotease inhibitor, 4-(2-carcoxyethyl) phenyl-trans-4-aminomethyl cyclohexane carboxylate hydrochloride (DV 1006) was studied in vitro, and compared with trans-4-aminomethyl cyclohexane carboxylic acid (AMCHA) and Trasylol. The results obtained were as follows: 1. The substance exerted a highly potent inhibiting activity on thrombin. The kinetic study employed a technique of Lineweaver and Burk revealed that the inhibition was competitive in fashion. 2. The substance inhibited either or both of process of the first phase of coagulation in which factors XI_α-IX and IX_α-X interactions were involved. 3. The substance was capable of to inhibiting both plasmin and plasminogen activation. A striking difference between DV 1006 and AMCHA was that the former inhibited casein digestion by plasmin, but the latter had esentially no effect. The caseinolysis induced by plasmin was inhibited competitively by DV 1006. 4. The AGLMelysis induced by urokinase was inhibited by DV 1006 and AMCHA at concentra tions of 10^-4M and 10^-2M, respectively. Trasylol had no effect on this reaction system. 5. The substance inhibited trypsin, seasprose and kallikrein, but did not inhibit chymotrypsin.