Using the ascites fractions of a cancer patient as an immunogen, we have recently prepared an interesting monoclonal antibody (MoAb), MUSE11 (IgG1), to a circulating pancreatic cancer-associated antigen (antigen MUSE11). Immunoperoxidase study revealed that antigen MUSE11 was detected in virtually all adenocarcinomas (especially pancreatic carcinoma), whereas it was either negative or faintly positive in their normal counterparts among the digestive organ tissues. Western blotting analysis and periodic acid-Schiff carbohydrate staining revealed that antigen MUSE11 is a Mr 300,000 glycoprotein that is clearly different from other high-molecular-weight glycoprotein antigens which have been used for detection of circulating antigens in pancreatic cancer patients. MoAb MUSE11 appears to react with a peptide epitope of this molecule, since periodic acid and neuraminidase treatment of the antigen did not affect the reactivity of MoAb MUSE11 with the antigen, but trypsin and protease treatment abolished the reactivity. The antigen MUSE11 was purified using affinity chromatography in conjunction with MoAb MUSE11. This antigen appears to be highly glycosilated with N-linked carbohydrate moieties, since treatment of the pancreatic carcinoma Panc-1 cells with tunicamycin reduced the molecular weight band to Mr 110,000. Once antigen MUSE11 has been purified by affinity chromatography, neither CA19-9 nor pancreatic oncofetal antigens can be detected, although they were present at very high levels in the crude ascitic starting material.