1. The phenomenon of phage production by one bacterial culture for another of different sort has been reproduced experimentally.
2. This phenomenon results from phage carried with the culture, and not from the spontaneous appearance of phage in a culture previously free from it.
3. Animals immunized against the lysogenic bacteria may develop antibodies that neutralize the phage carried.
4. The development of neutralizing antibodies on immunization with a bacterial culture is evidence of the presence of bacteriophage in the culture.
5. The failure of such antibodies to appear on immunization with bacteria does not necessarily indicate that bacteriophage is not present.