Previous histochemical studies have shown that changes occur in the composition of mucins both in preneoplastic and neoplastic lesions of the gastric mucosa. Since monoclonal antibodies are now available which recognize the protein product of distinct mucin genes, they are likely to provide useful tools for evaluating these changes. Thus, a monoclonal antibody 996/1 raised against a peptide epitope of the colonic mucin MUC2 was examined for its potential as a prognostic indicator in gastric cancer. 996/1 works well on formalin-fixed paraffin sections and shows good staining of the colonic goblet cells in the region of the golgi, while there is no staining of normal control gastric mucosa. The epitope was detected in all cases of intestinal metaplasia (44 samples) and some but not all cases of dysplasia (26 samples) and gastric carcinoma (74 samples). There was no significant difference between the positivity of the tumours according to their classification, stage and lymph node status. These results unfortunately gave little indication that this antibody would be a useful prognostic tool in gastric cancer. However, the pattern of 996/1 staining provides useful information about the molecular changes in mucin expression that occur in gastric carcinogenesis.