A special colony of albino rats was built up by selection and isolation from a population in which middle ear disease was highly prevalent. No cases of aural infection occurred in the selected group, whereas its precursor showed a crude incidence of 57 per cent. The subjection of selected rats to a rachitic diet and to overcrowding did not predispose to the development of middle ear disease.
The incidence of pneumonia was not similarly affected; thus, 52 per cent of the adult selected rats showed pulmonary lesions, and 78 per cent of the adult stock rats. There was, however, a significant reduction in the number of cases which showed advanced pulmonary lesions.
Certain theoretical considerations of middle ear disease and of pneumonia are discussed.