1. Single cell mouse strains of B. enteritidis from smooth-susceptible colonies were, under the various conditions tested, of the same high degree of virulence. 2 year old cultures, strains obtained both in interepidemic and epidemic periods, and "reverted" strains from variant colonies showed the same degree of pathogenicity.
2. Single cell strains from variant smooth, mucoid,and rough phage-resistant colonies were definitely less virulent than the usual smooth-susceptible strains. The loss of virulence in each instance resulted apparently from contact with bacteriophage which rendered the individual cells incapable of multiplication in the animal tissues.
3. A reduction of virulence was induced experimentally when washed cultures of the highly pathogenic smooth-susceptible cells were exposed to bacteriophage and thus rendered incapable of multiplication in the inoculated animals.