Salivary isoamylase was measured in duodenal aspirates of 124 consecutive patients during a secretin-pancreozymin test, using an inhibitor method. The isoenzyme was found in 22 patients (17.7%), more often in the patients with abnormal pancreatic function (40.5%) than in those with normal function (8.0%). Before the subtraction of the salivary isoamylase component, total amylase output was falsely normal in three of 37 patients with exocrine pancreatic insufficiency. It is concluded that measurement of amylase isoenzymes rather than of total amylase during the secretin-pancreozymin test may be recommended for scientific purposes. It may also improve discrimination between normal and abnormal exocrine pancreatic function if amylase is chosen as the only parameter for evaluation of pancreatic enzyme secretion.