The c-erb B-2 oncogene encodes a 190 kD transmembrane growth factor receptor which is closely related to the EGF receptor and has been found to be amplified and overexpressed in a number of human adenocarcinomas, particularly of the breast. We have analysed, by immunocytochemistry using the 21N antibody, expression of c-erb B-2 in a retrospective series of pancreatic adenocarcinoma, chronic pancreatitis, and examples of histologically normal pancreas. In three cases (21 per cent) of chronic pancreatitis, there were focal areas of cytoplasmic immunoreactivity in regenerating epithelium. In 15 cases (17 per cent) of pancreatic adenocarcinoma, cytoplasmic immunoreactivity was seen, while in two cases (2 per cent) strong membrane staining of tumour cells was seen which could be blocked by peptide controls. c-erb B-2 immunoreactivity was also demonstrated using a second antibody, 20N, which recognizes another peptide sequence of the c-erb B-2 protein. There was no relationship between immunoreactivity and histological subtype or grade, but there was absolute concordance between staining in primary and metastatic deposits. Since the rat homologue (neu) of the c-erb B-2 oncogene may be activated by a specific point mutation in its transmembrane region, we have analysed 23 cases from this series for mutations by polymerase chain reaction amplification and sequence-specific oligonucleotide hybridization. We were unable to identify activity mutations in this series. These data suggest that there is abnormal expression of c-erb B-2 oncogene in nearly 20 per cent of cases although mutational activation of this gene is not seen in human pancreatic adenocarcinoma.