1. Anti-Shiga bacteriophage injected into the general circulation can multiply in the brain of mice infected intracerebrally with Shigella dysenteriae. 2. Under proper conditions, the injection of active bacteriophage into the general circulation can protect mice against an otherwise fatal intracerebral infection with Shigella dysenteriae. 3. The protection so induced appears to depend upon the early establishment of a high bacteriophage level in the infected animal.