In order to investigate the expression of MUC5AC mucin in normal gastric mucosa and gastric carcinomas, we produced 3 monoclonal antibodies (MAbs) using a MUC5AC synthetic peptide. The immunohistochemical study was performed using one of these MAbs (CLH2) which reacted with the different designs of peptides based on the MUC5AC tandem repeat and with native and deglycosylated mucin extracted from gastric tissues. CLH2 immunoreactivity was restricted to foveolar and mucopeptic neck cells in normal gastric mucosa. No reactivity was observed in type-I intestinal metaplasia. Out of 66 gastric carcinomas, 42 (63.6%) expressed MUC5AC. Most diffuse carcinomas were positive (83.3%), whereas only 59.3% of intestinal and 40.0% of atypical carcinomas expressed MUC5AC (p < 0.05). Gastric carcinomas with mixed pattern showed immunoreactivity in diffuse areas and decreased immunoreactivity in intestinal areas. Every early gastric carcinoma expressed MUC5AC, in contrast to 58.6% of advanced carcinomas (p < 0.05). A trend toward decreased immunoreactivity was observed in deep areas of advanced carcinomas in comparison with the respective superficial areas. Taking together the specific staining of foveolar and mucopeptic neck cells and the absence of immunoreactivity in intestinal metaplasia, we conclude that MUC5AC expression may be used as a marker of gastric differentiation. This assumption is further supported by the finding of MUC5AC immunoreactivity in most diffuse carcinomas, which usually display morphologic and histochemical signs of gastric differentiation. The expression of MUC5AC in early gastric carcinomas, regardless of their histologic type, suggests that all gastric carcinomas retain at least some cells with a gastric phenotype during the first steps of neoplastic development.