A mucin preparation from a colonic adenocarcinoma was used to prepare monoclonal antibodies (MAbs) that reacted specifically either with normal adult small‐intestine mucin antigen(s) (SIMA), or normal adult large‐intestine mucin antigen(s) (LIMA). Both SIMA and LIMA show a unique oncofetal pattern of expression. Thus SIMA was expressed in early fetal stomach, large and small Intestines but thereafter only in the normal small intestine. SIMA expression was detected immunohistochemically in cancers of the colorectum (82/112) and stomach (48/86). LIMA was detected in the stomach of the early fetus but thereafter only in the normal large intestine. LIMA expression was detected in 61/86 cancers of the stomach. Moreover, both SIMA and LIMA were expressed inappropriately in mucosa adjacent to tumors, indicative of the detection of possible pre‐malignant epithelium. We used a sandwich ELISA and biochemical procedures to show that the SIMA and LIMA molecules were large extensively glycosylated multi‐unit mucin glycoproteins that differed markedly from each other. SIMA, whether extracted from normal small‐intestine or colonic cancers, had a molecular weight above 1.000 kDa, a mean buoyant density 1.33 g/ml and s value of 4.8. LIMA had a molecular weight above 10.000 kDa, a mean buoyant density 1.45 g/ml and an s value 9.5. The SIMA and LIMA epitopes were judged to be carbohydrate in nature by reason of their resistance to harsh physical chemical treatments or protease digestion, and sensitivity to periodate oxidation, neuraminidase or β elimination. Only the SIMA epitope was sensitive to neuraminidase. In conclusion, MAbs to carbohydrate‐dependent epitopes on SIMA and LIMA identify the oncofetal pattern of expression of these distinct intestinal mucin glycoproteins in colonic and gastric carcinoma. These MAbs will be useful in further studies of the significance of oncofetal mucin expression during carcinogenesis.