Four patients with erosive hemorrhagic gastroduodenitis were found to have high fibrinolytic activity of the gastric juice. No increase in the fibrinolytic activity could be demonstrated in the circulating blood, but the values found for fibrinogen, plasminogen and alpha2-macroglobulin were low. A high content of FDP was found in the serum. All patients had a markedly decreased content of factor XIII. Platelet count and other coagulation components were normal. These findings were interpreted as signs of local fibrinolysis in the diseased parts of the gastrointestinal canal. The bleeding stopped after oral and intravenous administration of a fibrinolytic inhibitor (AMCA Cyclokapron) and of factor XIII-containing concentrate. In bleeding from gastroduodenal ulcer and esophageal varices, no increase in gastric fibrinolytic activity was found. It is suggested that the high local fibrinolytic activity in the stomach in erosive gastritis together with the low content of factor XIII contributes substantially to the hemorrhage in this condition. These observations may lead to a revision of the treatment of such cases.