Summary The neutralization of bacteriophage R 17 was studied with rabbit antibody induced by R 17 and several modified antibody preparations derived from the γG-immunoglobulin fraction of the antiserum. To assess the role of bivalence and the Fc fragment in the neutralization reaction both 7 S and 5 S monovalent and bivalent antibody derivatives were tested as well as monovalent 3 S antibody. The bivalent preparations as a group gave K values at least 30-fold larger than the monovalent antibodies. Within each group there was little if any significant variation. The important role of bivalence, in contrast to the Fc fragment, is attributed to the increased affinity of the antibody for the bacteriophage arising from the bivalence of the antibody and the presence of multiple identical antigenic determinants on the bacteriophage. It is suggested that bivalence was the result of an evolutionary development whose selection was favored because of the increased protection it provided against infectious disease.