In a previous study, we obtained a novel monoclonal antibody (F1 alpha-75) directed against a synthetic mucin antigen termed F1 alpha, and demonstrated that this antigen was expressed in a high percentage (80.2%; 89/111) of gastric carcinomas. In the present study, we compared the expression of F1 alpha with that of sialyl-Tn antigen, a mucin antigen similar to F1 alpha, in 54 human early gastric carcinomas, intestinal metaplasia and adenomatous polyps of the stomach to determine how differences in the expression of these antigens correlated with gastric carcinogenesis. The rate of expression of F1 alpha in early gastric carcinoma tissues, 81.5%, was higher than that of sialyl-Tn antigen, 57.4%. No correlation was found between the rate of expression and histological type, depth of cancerous invasion or lymph node metastasis. Sialyl-Tn antigen was extensively expressed in 77.5% of specimens of complete intestinal metaplasia and in 78.6% of those of incomplete intestinal metaplasia; however, the expression of F1 alpha in those specimens was rare and sporadic, with rates of only 25.0 and 27.7%, respectively. In adenomatous polyps, the rate of expression of F1 alpha is 6.25% and that of sialyl-Tn antigen is 43.8%. Our findings indicate that F1 alpha is a more specific antigen for gastric cancer than sialyl-Tn antigen, and that F1 alpha is an antigen associated with carcinogenesis of the stomach.