Serum and urinary amylase isoenzymes were studied in an attempt to evaluate the usefulness of isoamylase analysis in the diagnosis of pancreatic diseases. The amylase in human serum and urine was separated into 4 to 5 distinct isoenzymes, which have mobilities consistent with those of the pancreatic isoamylases (Amylase-1, -2, -4 and -6) and the salivary isoamylases (Amylase-3, -5 and -7). The normal isoamylase pattern was Amylase-1, -2, -3 and -5. Amylase-4 and -6 could not be found in normal serum and urine. Amylase activity in Amylase-1 and/or -2 increased remarkably, having the normal isoamylase pattern, in the patients with elevated serum amylase activity after stimulation with pancreozymin-secretin or after endoscopic retrograde cholangiopancreatography. However, in the patients with acute pancreatitis, salivary type isoamylases disappeared and only the pancreatic type components were identified. It seems that the appearance of Amylase-4 and disappearance of the salivary components indicate severe pancreatic inflammation and that the apparent specificity of this isoenzyme pattern affords a mean for conforming the diagnosis of acute pancreatitis. However, the occurrence of an inherited trait in the pancreatic isoamylases of healthy individuals suggests that a rise in the pancreatic components may not necessarily indicate pancreatic disorders.