In combination with routine gastric secretion test and endoscopic congo red test, alkali labile pepsin (ALP) and alkali stable pepsin (ASP) were separately determined in total of 41 patients with peptic ulcer including 20 gastric (GU), 12 duodenal (DU) and 9 gastroduodenal ulcers (GDU), and in 10 normal controls. In both of basal and tetragastrin stimulated states, percent of ASP in total acid protease (%ASP) was increased in order of GU, GDU, normal control and DU with a close relation to the dimension of acid secreting area determined by endoscopic congo red test. Percent ASP was decreased after tetragastrin provocation in all disease groups. GU had high % ASP and low maximal acid output (MAO), GDU had high %ASP and high MAO, and DU showed low %ASP and high MAO.