The lysosomal protease cathepsin B is thought to play a crucial role in the intracellular activation cascade of digestive proteases and in the initiation of acute pancreatitis. Although cathepsin B has been shown to be physiologically present in the secretory pathway of pancreatic acinar cells it has been suggested that premature activation of zymogens requires an additional redistribution of cathepsin B into the secretory compartment. Here, we studied the role of cathepsin B targeting during caerulein-induced pancreatitis in mouse mutants lacking the cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (CI-MPR) which normally mediates the trafficking of cathepsin B to lysosomes. Absence of the CI-MPR led to redistribution of cathepsin B to the zymogen granule enriched subcellular fraction and to a substantial formation of large cytoplasmic vacuoles that contained both, trypsinogen and cathepsin B. However, this did not cause premature intracellular trypsin activation in saline-treated control animals lacking the CI-MPR. During caerulein-induced pancreatitis, trypsinogen activation in the pancreas of CI-MPR-deficient animals was about 40% higher than in wild-type animals but serum amylase levels were reduced and lung damage was unchanged. These data suggest that subcellular redistribution of cathepsin B, in itself, induces neither spontaneous trypsinogen activation nor pancreatitis. Furthermore, we clearly show that a marked increase in intracellular trypsinogen activation is not necessarily associated with greater disease severity.