The expression of large and small intestinal mucin antigens (LIMA and SIMA) was investigated in 30 gastrectomy specimens of carcinoma and in 11 controls resected for various pathologic conditions. One hundred eighty-five samples of normal mucosa, hyperplasia, intestinal metaplasia (types I, II, and III), dysplasia, and tumor were studied to identify phenotypes indicative of premalignant change. Our results showed LIMA and SIMA were not detected in normal gastric epithelium from either control or carcinoma specimens. SIMA characterized goblet cell mucin in all types of intestinal metaplasia and was not discriminatory between controls and carcinoma groups. On the other hand, LIMA was extensively expressed in columnar and goblet cells in carcinoma-bearing stomachs (97 per cent) but was absent in controls. There was a crescendo intensity and frequency of LIMA staining in an inverse relation to the degree of cell maturation and differentiation from type I intestinal metaplasia (60 per cent) to type II (85 per cent), type III (100 per cent), and dysplasia (100 per cent). In contrast, intestinal metaplasia of any type in controls did not show LIMA. The distribution of LIMA seemed to be intimately related to cell differentiation in the proliferative zone at the base of metaplastic glands. Carcinomas revealed antigenic phenotype heterogenicity. Our data indicate that LIMA sharpens the diagnosis of dysplasia, discriminates between reactive and preneoplastic epithelium (particularly within intestinal metaplasia), and detects abnormal phenotypes that may represent early stages in carcinogenesis.