Expression of blood-group antigens A, B, Le(a), sialyl-Le(a) (sLe(a)), Le(b), Le(x), Le(y), precursor type I, Tn and sialyl-Tn (sTn) was examined in non-neoplastic pancreas (n = 37) and pancreas cancer (n = 21) using mouse monoclonal antibodies (MAbs). Immunohistochemical assays were performed on sections of paraffin-embedded tissues using the avidin-biotin complex method. In normal pancreas, antibodies detecting Le(a), sLe(a) and Tn reacted with ductal epithelium, and antibodies detecting A and B reacted with acini and ducts, independently of secretor status. Le(x) was weakly expressed in ducts and acini, and sTn could not be detected in normal pancreas. Expression of Le(b), Le(y) and precursor type I was regulated by secretor status: Le(b) and Le(y) were expressed in ducts of secretor and Le(a-b-) individuals, but not in ducts of non-secretors; precursor type I was weakly expressed in acini and ducts of non-secretors and Le(a-b-) individuals, and was absent in acini and ducts of secretors. The following alterations in the expression of blood-group antigens were observed in pancreas cancer: (1) enhanced expression of Le(x), Tn and sTn; (2) enhanced expression of precursor type I independently of secretor status; (3) loss of regulation of Le(b) by the secretor gene; (4) decreased expression of Le(y). The weak expression of precursor type I. Tn and sTn in non-neoplastic pancreas, and their stronger expression in pancreas cancer, suggests that up-regulation of their expression is associated with malignant transformation of pancreatic duct cells.