Trypsinized human erythrocytes were found to bind 3 times as much soybean agglutinin as the untreated cells. With rabbit erythrocytes, no effect of trypsinization on the binding of the agglutinin was observed. However, the susceptibility to agglutination increased 100- to 200 fold for all erythrocytes. The increased erythrocyte agglutinability can therefore not be explained simply by an increase in the number of agglutinin binding sites.