Human gastric mucins were chemically deglycosylated with trifluoromethanesulfonic acid. A mouse monoclonal antibody (MoAb), SO-MU1, was established against the deglycosylated mucins. SO-MU1 recognized not only the deglycosylated mucins but also the native gastric mucins. Periodate treatment of the native mucins increased the SO-MU1 reactivity. Trypsin digestion abolished the antigenicity. Human gastric cDNA expression library was screened with SO-MU1 and a mucin cDNA clone was obtained. Its sequence contained the MUC5AC tandem repeat domain. We studied gastrointestinal distribution of the SO-MU1-reactive antigen immunohistochemically. Gastric surface epithelial cells and parietal cells expressed the antigen, but the glandular cells did not. The antigen was also detected in the small intestine and biliary tract but not in the colon and pancreas. In summary, (1) MoAb SO-MU1 was raised against human gastric mucins, (2) it recognized human gastric MUC5AC apomucin, and (3) the SO-MU1-reactive antigen showed characteristic distribution in the organs of endodermal origin. MoAb SO-MU1 is the first MoAb against MUC5AC.