The infective capacity of three bacteria commonly encountered during a study of natural middle ear disease in a rat colony has been determined by direct intra-aural injection in young rats. One week after the introduction of B. actinoides 75 per cent of the rats showed a purulent exudate in the injected middle ear cavity and 65 per cent yielded pure cultures of the organism. With hemolytic and non-hemolytic streptococci 75 per cent showed a serous or mucoid exudate and 12 per cent yielded the organism in culture. With a diphtheroid 18 per cent showed a gross reaction in the middle ear which was sterile in every case.
The experimental findings are discussed in relation to the etiology of middle ear disease.