1. Rabbits vaccinated with tubercle bacilli killed by exposure to formalin (0.4 per cent) did not show any acquired resistance to subsequent infection with bovine tubercle bacilli, while rabbits vaccinated with tubercle bacilli which had been killed by heating to 70° for 1 hour survived more than half as long again as their controls.
2. Intraperitoneal injection of either the formalin-killed vaccine or the heat-killed vaccine into guinea pigs made them skin-sensitive to tuberculoprotein MA-100.
3. The rate of absorption of the formalin-killed vaccine when introduced beneath the the skin was similar to that of the heat-killed vaccine.
4. Following the intravenous injection of heat-killed tubercle bacilli, it was found that rabbits developed a massive tubercular pneumonia. A study of the production and ultimate absorption of the cellular exudate showed that these processes were similar to those found after the injection of living bacilli. The lesions which followed the injection of heat-killed bacilli differed from the lesions found in active tuberculosis in that in any one animal they showed a striking uniformity in appearance, while in the active disease the lungs showed a great diversity in type of lesion. Studies of the blood cells during the period of injection of dead organisms showed that the changes which are characteristic of the period during which a tuberculous pneumonia develops in rabbits (30 to 40 days after inoculation) were faithfully reproduced. It is suggested that the process of regression described may be similar to that which occurs in childhood tuberculosis, in which rather extensive pulmonary lesions resolve without leaving evidence of damage to the parenchyma of the lung.