SDS electrophoresis on polyacrylamide gels of purified trypsinogen 1 has shown the occurrence of a proteolysis in some molecules during long storage at -20 degrees C. This proteolyzed trypsinogen gives a positive reaction with an antiserum directed against the precipitate protein, major protein of about 14 000 molecular weight extracted from precipitates present in the pancreatic juice of patients with chronic pancreatitis. The autoactivation of proteolyzed trypsinogen 1 liberates a polypeptide of 14 000 molecular weight which is immunologically identical to the precipitate protein. These results show that the major protein present in pancreatic precipitates (and pancreatic stones) of patients with chronic pancreatitis is a degradation product of trypsinogen 1 liberated by a proteolysis which necessarily requires a premature zymogen activation in the disease.