Increased sialylation of cell surface glycoconjugates has been demonstrated in malignant tumors and shown to be correlated with the invasive and metastatic growth of colon carcinoma cells. The authors have applied the Maackia amurensis lectin, which interacts with alpha 2,3-linked sialic acid, and the Sambucus nigra I lectin specific for alpha 2,6-linked sialic acid. In human colon, alpha 2,3-linked sialic acid was detectable in normal and transitional mucosa as well as in adenomas with different degrees of dysplasia and in carcinoma. In contrast, alpha 2,6-linked sialic acid as visualized with Sambucus nigra I lectin was found only in severe dysplasia and carcinoma. Thus expression of binding sites for Sambucus nigra I lectin was associated with the occurrence of histologic features of malignancy. It is concluded that malignant transformation in human colonic epithelium is accompanied by the de novo expression of an alpha 2,6 sialyl-transferase. These findings provide the basis for more detailed studies of the possible role of cell surface glycoconjugates bearing alpha 2,6-linked sialic acid in growth behavior of human colonic epithelial cells.