Immunoreactive trypsin in serum can be divided into trypsinogen and trypsin-alpha 1-proteinase inhibitor (alpha 1PI) complexes. These were studied separately in serum from 204 patients with acute gastro-intestinal symptoms. Elevated levels of both trypsinogen and trypsin-alpha 1PI complexes were seen in patients with acute pancreatitis. Elevated levels of trypsinogen and normal or slightly elevated levels of trypsin-alpha 1PI complexes were seen in patients with biliary tract diseases. An isolated increase in the concentration of trypsin-alpha 1PI complexes with normal trypsinogen and amylase levels were seen in patients with perforated ulcer. This third cluster may result from an absorption of active trypsin from the peritoneal cavity. Small amounts of trypsin-alpha 1PI complexes were present also in serum from patients free from pancreatic disease. The results in this study show that high levels of trypsin-alpha 1PI complexes in serum are seen mainly in patients with acute pancreatitis. However, elevated levels are also seen in other pathological conditions in the upper gastrointestinal tract; therefore an assay for these complexes is not a specific diagnostic test for acute pancreatitis.