Toxic substances produced in hemorrhagic ascitic fluid during experimental hemorrhagic pancreatitis in dogs were investigated. An average of 394 ml of ascitic fluid was produced within 5 h after the induction of acute pancreatitis by intraductal injection of a mixture of autologous bile and trypsin. Hemorrhagic ascitic fluid was collected under sterile conditions, which was confirmed by aerobic and anaerobic culture and a Limulus test. The sterile fluid was injected intraperitoneally into mice in doses of 2 and 3 ml, and the mortality rate 72 h after injection was 66.0 and 88.4%, respectively. It contained high concentrations of pancreatic enzymes, including trypsin and esterase activity, as well as bradykinin, histamine and prostaglandin. Autopsy and histological examination of mice revealed shock with lung damage. The results suggest that hemorrhagic ascitic fluid produced in pancreatitis may be an important factor for early deaths in acute pancreatitis. When a new synthetic antiprotease (nafamstat mesilate) in a dosage of 0.2 mg was mixed with 1 ml of ascitic fluid, trypsin was not detectable, and bradykinin was reduced 1.0 ng/ml from 8.0 ng/ml, while esterase activity decreased to one tenth of its previous activity. The mortality following injection of the solution decreased to 26.7 and 80.6%, respectively. These results indicate that peritoneal lavage with a solution containing antiprotease may be an effective treatment for hemorrhagic acute pancreatitis.