Isoamylase analysis by isoelectric focusing was performed in the serum of 30 healthy volunteers, 65 patients with acute or chronic pancreatic diseases, nine with acute abdomen, four with macroamylasemia, and four with duodenal duplication. In controls, up to four fractions (2 salivary, 2 pancreatic) were found; the pancreatic fractions were as a mean 44.7% (SD 8.6) of total. In chronic pancreatitis, only patients with steatorrhea showed a significant reduction of pancreatic isoamylase (p less than 0.001). In all patients with acute pancreatitis or pseudocysts, an additional fraction (similar to the so-called P3 fraction) was resolved. Moreover, additional isoenzymes were found in all patients with severe acute pancreatitis or pseudocysts, and not in controls or patients with mild forms, acute abdomen or duodenal duplication. A similar pattern was shown in a stored control serum after 10 mo at -20 degrees C. These fractions disappeared after successful surgical drainage. No specific alteration was found in pancreatic cancer. Amylase fractionation by isoelectric focusing can be used to confirm an acute pancreatitis, and to monitor patients with pancreatic pseudocysts and collections after surgical drainage.