Acinar cell carcinoma (ACC) is a rare pancreatic cancer which may be difficult to distinguish from other solid nonadenocarcinoma tumors. The diagnosis depends on the demonstration of acinar differentiation, obtained with antibodies recognizing various pancreatic enzymes that, although specific, show different sensitivity. The C-terminal portion of the BCL10 protein shows homology with carboxyl ester hydrolase (CEH), an enzyme produced by pancreatic acinar cells. We investigated the usefulness of a C-terminal BCL10 monoclonal antibody in the diagnosis of ACCs. We examined normal pancreases and different pancreatic tumors including ACCs, mixed acinar-endocrine carcinomas, ductal adenocarcinomas, mucinous, serous, solid pseudopapillary, and endocrine neoplasms. In addition, various normal tissues and cases of pancreatic metaplasia of the gastroesophageal mucosa, cases of ectopic pancreas, gastrointestinal endocrine tumors, salivary and breast acinic cell carcinomas, gastric adenocarcinomas with and without acinar differentiation, and hepatocellular carcinomas were studied. BCL10 immunoreactivity paralleled that of CEH and was restricted to acinar cells of normal and ectopic pancreas, of pancreatic metaplasia, and of ACCs. The anti-BCL10 antibody was more sensitive in detecting ACCs and pancreatic metaplasia than antibodies directed against other pancreatic enzymes. We suggest using BCL10 antibody for diagnosing pancreatic tumors and whenever an acinar differentiation is suspected in gastrointestinal neoplastic and metaplastic lesions.