AbstractMonoclonal antibody 19–9 produced by a hybridoma prepared from spleen cells of a mouse immunized with a human colon carcinoma cell line, detects an antigen in sera from many patients with gastrointestinal and pancreatic cancer. This antibody is directed against a carbohydrate with the sugar sequenceNeuNAcα2–3Galβ1‐3GlcNAcα1–3Gal…4 1 Fucα1In many gastrointestinal and pancreatic cancers this sequence occurs in a monosialoganglioside containing a sialylated Lea‐active pentasaccharide (sialylated lacto‐N‐fucopentaose II, IV3‐α‐NeuNAc‐III4‐α‐Fuc‐LcOse4). In the sera of cancer patients the antigen occurs as a mucin. The antigen is also found in salivary mucin from many normal individuals belonging to the Le(a+b‐) or Le(a‐b+) blood group but not in salivary mucin from normal individuals belonging to the Le(a‐b‐) blood group. There is more antigen in mucin from individuals belonging to the Le(a+b‐) blood group than in mucin from indivi