Expression of sulfated carbohydrate chains in intestinal metaplasia (IM) and gastric cancer tissues was immunohistochemically evaluated by using a monoclonal antibody (MAb) 91.9H. While normal gastric tissues were negative for MAb 91.9H, IM and cancer tissues were positively stained with high frequency. The incidence was 67% for IM with chronic gastritis (n = 12), 95% for IM with gastric cancer (n = 21), 77% for well and moderately differentiated adenocarcinoma (n = 13), 48% for poorly differentiated adenocarcinoma (n = 23), 89% for signet-ring cell carcinoma (n = 9) and 100% for mucinous adenocarcinoma (n = 7). When poorly differentiated adenocarcinoma cases were divided into two groups, solid (n = 13) and non-solid types (n = 10), the incidences were 8% and 100%, respectively. These data suggest that MAb 91.9H could be of practical use as a new marker for IM and gastric cancer, and may be valuable for subgrouping poorly differentiated adenocarcinomas. Analyses of the core proteins for 91.9H epitope were then carried out. Comparison of immunostaining of ten poorly differentiated adenocarcinoma cases by MAb MUSE11 against MUC1 gene product with that by MAb 91.9H suggested that 91.9H epitope is not expressed on MUC1. Northern blot analysis of 10 pairs of gastric cancer and adjacent normal tissues with a MUC2 cDNA probe showed that the expression level of MUC2 mRNA was below the limit of detection. Thus, 91.9H epitope may be expressed on other proteins than MUC1 or MUC2 core proteins in gastric cancer.