1. Guinea pigs living in the same room but not in the same cage with tuberculous animals acquire tuberculosis, characterized by a chronic course, a marked involvement of the lungs, often with cavity formation and a massive tuberculosis of the tracheobronchial nodes; the mesenteric and cervical nodes are slightly or not at all affected.
2. The route of infection in these guinea pigs is almost always the respiratory tract.
3. Of 103 guinea pigs exposed for a period of up to 32 months 15 or 14.5 per cent developed tuberculosis. The shortest period of exposure leading to fatal tuberculosis was 8 months.
4. The incidence of this tuberculosis acquired by air-borne contagion increases with the duration and intensity of the exposure up to a certain point.
5. A large percentage of the guinea pigs weathered a continuous exposure to the tubercle bacillus for 32 months without becoming tuberculous. This may be due to an innate natural resistance against tuberculosis, or to an acquired immunity resulting from the continuous exposure to the contagion.