A variety of typical agglutinating δG antibodies agglutinated papain-treated cells in a uniform manner in about 16-fold higher dilutions than nontreated cells. With δM antibodies the corresponding titer increase was on the average 4-fold. The papain-treatment of cells also increased the titers of cold agglutinins, phytohemagglutinins and preparations of PVP. Antiglobulin consumption experiments revealed that no more antibody was fixed from the same volume of antiserum onto papain-treated than onto nontreated cells. Possible mechanisms underlying enhanced agglutination of enzyme-treated cells were discussed. It is suggested that the same type of antibody may react with papaintreated and nontreated cells but that a smaller number of antibody molecules are needed for agglutination of papain-treated cells because of altered surface properties of treated erythrocytes.