To understand the function of pancreatic zymogen granules, we performed a proteomics analysis to identify ZG membrane components. Here we report the identification of Rab27b through this proteomics study and validate its role in granule function. MALDI-MS peptide mass fingerprint was matched to rat Rab27b with 43% sequence coverage, and the identification was also confirmed by tandem mass spectrometry. The localization of Rab27b on ZGs was confirmed by Western blotting and immunocytochemistry. To examine the function of Rab27b in acinar secretion, we overexpressed wild type and mutant Rab27b protein in pancreatic acini using recombinant adenoviruses. Wild type Rab27b had no effect on amylase secretion, while Rab27b Q78L enhanced, and Rab27b N133I inhibited, CCK-induced amylase release by 92+/-13% and 53+/-8%, respectively. This enhancement and inhibition occurred at all points on the CCK dose-response curve and over a 30min time course. These results demonstrate that Rab27b is present on ZGs and plays an important role in regulating acinar exocytosis.