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Second generation anti-peptide monoclonal antibodies were produced using synthetic peptide SIB35 (C-HSTPSFTSSITTTETTSHSTPSFTSSITTTETTS) as an antigen which contains two of the MUC3 tandem repeats. Three monoclonal antibodies were produced (M3.1, M3.2 and M3.3) which reacted with the immunizing peptide, but also reacted strongly with colon tissues. Using the immunoperoxidase staining technique, two of the monoclonal antibodies, M3.1 and M3.2, reacted with both colon carcinoma and normal colon tissue whereas the antibody M3.3 reacted with normal colon tissues but very weakly, if not at all, with colon carcinoma. MUC3 was found to be distributed in colon and rectum, and was also present to a lesser extent in breast, lung and salivary gland tissues. Analysis of mucin molecules by Western blotting, revealed the antigen detected by the antibodies (M3.1 and M3.2) to be of a high relative molecular mass. M3.1 and M3.3 reacted with eptiopes SITTTE and PSFTSS, respectively. M3.2 did not react with any of the 6-mer peptides, even though it reacted with the full length VNTR (2 repeat peptide–SIB35). Anti-MUC3 peptide antibodies appear to react with colon, rectum, breast, salivary gland and lung tissues, and represent a new method of producing anti-tumour antibodies.