Although it is well established that amylase activity is inactivated by acid, little is known about the influence of native gastric juice on amylase activity. We investigated the effect of gastric juice, acid, and pepsin on both pancreatic (P-) and salivary (S-) isoamylase activities in vitro. S- and P-isoamylase activities were completely inactivated by HCl (10 mEq/l), but 20% of each isoamylase activity remained when bovine serum albumin (BSA) (3%) was added. However pepsin (0.2 tyrosine mg/ml/min) inactivated both isoamylase activities even though BSA was added. When saliva or pancreatic juice was mixed with gastric juice, both isoamylase activities were detected until the mixing ratio was 3:7 (gastric juice:saliva or pancreatic juice, v/v). It is suggested that some of both isoamylase activities could survive after exposure to gastric juice in the stomach and duodenum, depending on the concentrations of acid and protein in gastric juice, and mixing ratio to gastric juice.