The expression of the erbB-2 gene product was studied immunohistochemically on 38 normal colonic mucosae, 14 adenomas and 44 colon cancers, with the use of two anti-erbB-2 antibodies, a rabbit polyclonal antibody specific for the intracellular domain and a mouse monoclonal antibody specific for the extracellular domain. Normal mucosae and adenomas were not stained. Five cases (11%) of colon cancer were positive with the polyclonal anti-erbB-2 antibody, while only one case was positive with the monoclonal antibody. Most of the positive cases were in Dukes C stage. The rare overexpression of the erbB-2 protein in colon cancer seems to indicate a minor role for the gene in colorectal tumorigenesis.