CA 19-9, a carbohydrate epitope expressed on a high MW (>400kDa) mucin glycoprotein, is a sialyl Lewis a structure which is synthesized from type 1 blood group precursor chains and is present in individuals expressing the Lewisa and/or Lewis b  blood group antigens. In normal tissues, sialyl Lewisa antigen is present in ductal epithelium of the breast, kidney, salivary gland, and sweat glands. Its expression is greatly enhanced in serum as well as in the majority of tumor cells in gastrointestinal (GI) carcinomas, including adenocarcinomas of the stomach, intestine, and pancreas. Preoperative elevated CA19-9 levels in patients with stage I pancreatic carcinoma decrease to normal values following surgery. When used serially, CA19-9 can predict recurrence of disease prior to radiographic or clinical findings. This MAb is superb for staining of formalin-fixed, paraffin-embedded tissues.