A colony of albino rats was developed from three breeders (not germ-free) originally delivered by Cesarian section and artificially reared in the laboratories of Professor James A Reyniers.

The colony has been maintained under quarantine since June, 1948, in the absence of any demonstrable involvement of the respiratory tract or of the middle ears.

At autopsy pleuropneumonia-like organisms, Streptobacillus moniliformis, and the virus-like agent associated with endemic pneumonia were not recoverable from the lungs or the nasal passages of 81 young and adult rats.

During the same time interval the morbidity rate of pneumonia in another rat colony known to be free from infectious catarrh was 51 per cent while the incidence of infection with the virus-like agent approached 100 per cent.

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