The tumor marker CA242™ is defined by the monoclonal antibody C242™. The chemical structure of the antigenic determinant is not exactly known, but the determinant has been shown to be a sialylated carbohydrate structure. In serum, CA242™ is found on the same mucin complex as CA50 and sialylated Lewis A (CA19-9). Thus, CA242™ is related, but not identical to the epitope of CA19-9. Serum levels of CA242 are low in healthy subjects and subjects with benign diseases, while elevated levels are commonly found in serum from patients with gastrointestinal cancer. The CA242™ marker may be used as an aid in the diagnosis and management of patients with known or suspected gastrointestinal carcinomas. The CanAg Diagnostics CA242 EIA should not be used as a substitute for any established clinical examination of malignancy, but may be used as a complement to existing clinical and laborator methods.