Carbohydrates of the epithelial cell membrane are involved in cell-cell and cell-substrate interaction, and changes are seen in relationship to cell differentiation and neoplastic transformation. The terminal part of carbohydrate structures carried on oral epithelial cells often expresses antigens of the ABO and Lewis blood group systems. The expression of these antigens are in oral mucosa genetically regulated by the A, B, H, Lewis, and secretor genes with subsequent correspondence between the blood group antigens expressed on erythrocytes and on oral epithelial cells. Variation in expression of carbohydrates is also seen in relationship to terminal differentiation in that blood group antigens and their immediate precursor structures are sequentially expressed on cells during their pathway through the epithelium. Various organs and tissues differ in their expression of cell surface carbohydrates. In oral mucosa, a close relationship is seen between the type of tissue differentiation and expression of blood group antigen; keratinized, nonkeratinized, and junctional epithelium all show different patterns of carbohydrate expression.