A large proportion of the investigations dealing with bacteriophage have been concerned with the isolation of an active lytic principle against many different strains of bacteria. These studies, in general, have varied but little from the classic experiments of d'Herelle dealing with the isolation and development of several true bacteriophages (1917). This phenomenon is now so well known to most students of bacteriology that there are relatively few laboratories where the reaction between a susceptible strain of bacteria and a potent bacteriophage has not been observed. Since the initial discovery was described an enormous literature has accumulated bearing upon the relationship between bacteria and bacteriophage. As early as 1920 d'Herelle had shown that this infravisible being is responsible for bringing about marked changes in bacteria. Subsequently, he, as well as many others, have studied the transformations of various bacterial species brought about through the agency of bacteriophage.