Neuraminidase treatment made cattle red cells (CRC) agglutinable by the agglutinins of the different heterologous but not of the homologous sera. These agglutinins were, however, absorbed by both the nauraminidase-treated and the intact CRC.Proteolytic treatment made CRC aggIutinable also by the norma1 cattle, isoimmune and autologous sera. Agglutination titres of the CRC ranged from 1:2 to 1:256, but the variation between CRC from members of monozygous (MZ) pairs was not greater than zk 2 agglutination score units the range of experimental error.Treatment with trypsin made the A,,A, factors more emergent on the surface of CRC for agglutination by anti-A,,while pronase treatment had a similar effect upon aggIutination of Z-positive cells by anti-Z. r m