A simple procedure for covalent coupling of polysaccharides (PS) to erythrocytes for passive hemagglutination is described. The PS is dissolved in saline and activated by adding cyanogen bromide, keeping the pH at 10.7. After adjusting the pH to 7.8, the activated PS solution is admixed to a 2% erythrocyte suspension for coupling, for 2 hr. Using dextran fractions for coupling, such sensitized cells show a high degree of specific sensitivity, similar to that produced by contact with stearoyl-dextran. Another PS, guaran, was similarly coupled to erythrocytes, conferring on them a high degree of specific sensitivity.