In order to develop reagents that can detect the exposed core carbohydrate antigens of mucins, we have prepared monoclonal antibodies against partially deglycosylated LS174T human colon cancer mucin. The three monoclonal antibodies, 10F4, 15D3a, and 91S8, stained cancers of the colon, pancreas, stomach, breast, prostate, and lung to a greater extent than corresponding normal tissues. There was no staining of normal pancreas or breast, suggesting that these antibodies may be particularly useful for detecting cancers in these two organs. In homogenates of cultured cancer cells, antigen was detectable in three colon cancer cell lines, but not in a variety of other epithelial cancers. The epitope specificity of all three monoclonal antibodies appears to be for Tn antigen, i.e. GalNAc-alpha-Ser/Thr, based on their recognition of alpha-linked GalNAc, but not T antigen, sialyl Tn, or a range of other structures. However, the three anti-Tn antibodies differed in tissue staining specificity and in relative binding to different mucins. These monoclonal antibodies, prepared against deglycosylated colon cancer mucin, appear to be useful reagents for the immunohistochemical detection of epithelial cancers, especially pancreatic cancer and breast cancer.