THERE is considerable evidence to suggest the involvement of cell surface carbohydrates in cell-cell recognition and adhesion in, for example, muscle cell fusion1 and the adherence of bacteria to cells2. It is not yet clear whether the selectivity of the recognition process is a consequence of the structure and sequence of the saccharide chain and/or the topographical distribution of the saccharide unit(s) in the membrane. Further insight into the relationship between cellular specificity and carbohydrate structure and distribution may be gained if it were possible to introduce saccharide molecules on to specific portions of the cell surface. We show here that bovine erythrocytes, which are not normally agglutinated by the plant lectin concanavalin A (con A) (refs 3, 4), can be rendered agglutinable by the chemical introduction on to the cell surface of α-D-mannose residues, which are thought to be the primary specificity determinant for this lectin. The lectin-induced agglutination of cells can be taken as a model for the investigation of the general problem of the role of sugars in cell–cell recognition.