The concentration in serum of cathodal trypsinogen has been studied in certain clinical and experimental situations. The concentration correlated with pancreatic amylase activity. Low levels were found in patients with malabsorption due to exocrine pancreatic insufficiency. The concentration rose after endoscopic retrograde cholangiopancreatographic examinations (ERCP). After ERCP, however, no trypsin was detected complexed with protease inhibitors, as is generally found in acute pancreatitis. The trypsinogen concentration in serum also rose in renal failure indicating a renal elimination route for the endogenous trypsinogen.