Comparison of a new immunoassay for determining serum pancreatic isoamylase with two standard techniques. A method has recently been developed for measuring serum pancreatic (P) isoamylase, using two monoclonal antibodies specific for salivary isoamylase. We performed this test on 67 healthy controls and 133 patients: 15 with acute pancreatitis, 53 with chronic pancreatitis (20 during painful relapse and 33 in clinical remission), 18 with pancreatic cancer, 41 with nonpancreatic disease with abdominal pain, five with macroamylasemia, and one with total pancreatectomy. Results were compared with those of a wheat germ inhibition method and with electrophoresis on cellulose acetate. A close correlation was found between the results of immunoinhibition assay and those of the other two tests. All patients with acute pancreatitis had abnormally high values in all three tests. In the group with chronic pancreatitis studied during painful relapse, 16 had an increase in P-isoamylase, as determined with the immunoinhibition assay, 13 with the wheat germ inhibition test, and 15 with electrophoresis. In the group with chronic pancreatitis in clinical remission, we found low values in one patient, by immunoinhibition assay, but found low values in 17 and 19 patients by wheat germ inhibition and electrophoresis, respectively. Low P-isoamylase values corresponded to a severe exocrine pancreatic insufficiency. In the group with pancreatic cancer, the three tests showed similar results, and the majority of the patients had normal values. In the patients with nonpancreatic diseases, abnormally high levels were found in five, by immunoassay, in four by electrophoresis, and in three by the wheat germ inhibition method. In the five cases with macroamylasemia, both inhibition assays erroneously demonstrated an abnormal P-isoamylase elevation. The results show that the three tests are equally useful for the diagnosis of acute pancreatitis, or chronic pancreatitis during an acute relapse. In these diseases, the immunoinhibition test would be the preferred assay because it is simple and rapidly performed. 