Coccobacilliform bodies, pleuropneumonia-like organisms, or other cultivable pathogenic bacteria were not demonstrable in lung and exudate suspensions from selected adult rats naturally infected with endemic pneumonia or from experimentally infected mice.

The specific factor in lung suspensions from mice was identified with an agent which was active on dilution through 107. It failed to pass through Berkefeld N filters but was generally present in V filtrates, with a loss in titer up to 1000-fold. It was largely removed from suspension on centrifugation at a speed of 9000 R.P.M. for 30 minutes.

Attempts to cultivate the agent in embryonated eggs were uniformly unsuccessful. In fluid suspension it failed to withstand storage at 40°C. for a week but remained viable for at least 3 months in a frozen state under dry ice.

The relation of the infective agent to viruses and to pleuropneumonia-like organisms is discussed.

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