Small Gram-negative cells resembling the so called coccobacilliform bodies of fowl coryza were regularly found in the nasal and middle ear exudate of mice naturally and experimentally infected with catarrh. These bodies were successfully isolated from exudates and cultivated in tissue cultures. There was no microscopic evidence, however, of their multiplication in ordinary nutrient media enriched with blood. They were filterable through collodion membranes with an average pore size of 640 mµ and, hence, separable from secondary bacteria. The size of the bodies in stained films averaged between 0.3 and 0.4µ.
A second organism cultivable in fluid blood media with the formation of compact clumps and similar to the X bacillus of chickens was also isolated from infected mice.