Considerable alteration of cellular carbohydrates such as glycolipids and glycoproteins occurs in colonic neoplasia. Some of these changes are also observed at certain embryonic stages of differentiation and are, therefore, considered onco-developmental changes. In colon cancer cells, many of the phenotypic markers for malignancy have been found on carbohydrate moieties, and some have been found on the peptide portion of mucin glycoproteins. The changes in carbohydrate antigens include altered expression of core region carbohydrates, extension of backbone structures and modification of peripheral carbohydrate structures that may arise due to abnormal glycosylation processes. Altered glycosylation may also result in the exposure of the peptide moiety of the mucin glycoprotein. Therefore, these altered mucin glycoprotein structures may serve as tumor markers. However, it remains to be determined whether they will be useful as intermediate endpoint markers.