The asialocarbohydrate antigen YH206 is expressed on adenocarcinoma-associated mucin molecules which lack epitopes of CA19-9 and DU-PAN-2. To further characterize this molecule, the monoclonal antibody BM2 against the affinity-purified antigen YH206 was established. It was demonstrated by an inhibition test that antigen BM2 was an X-hapten-like structure, one of the representative oncodevelopmental antigens. Although the sensitivity of antigen BM2 in sera of stomach and pancreas cancer patients did not appear to be superior to that of antigen YH206, both antigens were complementary to each other resulting in the improvement of sensitivity. Interestingly, double-determinant enzyme immunoassays showed that antigen BM2 and YH206, both having a cryptic nature for neuraminidase, were co-expressed on the same mucin molecule in sera of patients with stomach cancer or liver cirrhosis. These data suggest that mucin molecules in serum might be classified into several groups based on the distribution of tumor-associated epitopes.