Abstract
1. A spontaneous epidemic of Gaertner infection among rats was found associated with methaemoglobinaemia and, in some cases, anaemia. 2. Strains of Gaertner's bacillus isolated from these rats reproduced methaemoglobinaemia in fresh rats but not in rabbits, guinea-pigs or mice. 3. Other strains of Gaertner's bacillus from rats, guinea-pigs and human sources also caused methaemoglobinaemia either before or after passage through rats. 4. Other organisms pathogenic for rats did not produce methaemoglobinaemia.