In 1981, a new low-molecular-weight protease inhibitor, FUT-175 (nafamstat mesilate), was synthesized. Its preventive action against acute experimental pancreatitis (AEP) was detected. We studied the effect of FUT-175 on the blood count and aggregability of platelets in AEP in dogs. At 30 min after induction of AEP, the sensitivity to ADP increased more than two times in untreated animals. An evident decrease in platelet count of about 37% was noted in these dogs at 6 h after AEP induction. Treatment of AEP with FUT-175 prevented these changes. We assume that the positive effect of FUT-175 against AEP depends at least in part on its influence on platelet aggregation.