(ii) Efficacy. Professor Wilson of the Emergency Public Health Laboratory Service, Oxford, has recently pointed out that although among a rat population that has not previously been exposed to it the 'virus' may kill a large proportion, the survivors acquire immunity. When experimentally the survivors of some mice which had been infected with a highly virulent organism closely related to Ratin bacillus were allowed to breed, they developed in each generation an increased resistance to infection with this organism.