The location of carbonic anhydrase (CA) isoenzymes I, II and VI in normal and neoplastic pancreatic tissue was studied using polyclonal antisera and the immunoperoxidase technique. Samples were obtained from patients with well-differentiated (n = 4), moderately differentiated (n = 1) and poorly differentiated (n = 4) ductal adenocarcinomas, cystadenocarcinoma (n = 2), adenosquamous carcinoma (n = 1), acinar adenocarcinoma (n = 1), gastrinoma (n = 3), insulinoma (n = 3) and glucagonoma (n = 1). The control specimens were from a patient with traumatic laceration of the pancreas. The normal and malignant endocrine tissue showed intense positive staining for CA I localized in the cells expressing glucagon. In the exocrine pancreatic tissue, CA II was detected in the normal and neoplastic ductal epithelium. No specific staining was detected with anti-CA VI serum in either normal or malignant tissue.