1. The experiments here reported were carried out with a lytic principle isolated from the feces of normal rabbits.

2. It seems that bacteriophage is constantly present in the feces of caged laboratory rabbits.

3. A lytic filtrate active for Strain Shiga 73 (in a dilution of 1:1,000,000) and for the typhoid bacillus (in a dilution of 1:1,000), after twenty successive passages with Shiga bacillus was active, for its homologous strain, in a dilution of about 1:10,000,000,000 but no longer possessed any lytic activity for the typhoid strain. The same filtrate, after twenty successive passages with typhoid bacillus, was similarly active in a dilution of 1:10,000,000,000 for the typhoid strain but was only feebly active for the Shiga strain.

4. The passage of the lytic principle through collodion membranes was never observed.

5. No appreciable diminution in the activity of the filtrates was observed after exposure to cholesterol or lecithin. Killed bacterial suspensions, however, were found to exert a considerable neutralizing effect on the active principle.

6. Formol titrations of lysed cultures lead to the conclusion that the bacteriolysis caused by the bacteriophage is not a true proteolysis but rather a simple plasmolysis of the bacterial cell and not a splitting effect of the protein molecule.

7. We have never obtained active filtrates with the leucocytes of immune animals by the method of Lisbonne, Boulet, and Carrère (14).

8. It would appear, from the experiments, that the immunization of rabbits with the lytic principle results in the disappearance of the bacteriophage from their intestines.

9. All the sera from rabbits immunized with antityphoid or antiShiga filtrates agglutinated respectively the typhoid or Shiga bacilli.

10. Similarly, these sera fixed complement strongly in the presence of the homologous antigen. Five or six times the amount of serum was required to fix complement in the presence of heterologous antigen.

11. All of the immune sera were found to be more or less antilytic. In no case was a strictly specific antilysin obtained.

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