The course of pulmonary tuberculosis was followed in control and BCG-vaccinated mice. Resistance was evaluated by determining the numbers of virulent mycobacteria recovered from the organs at various intervals after airborn infection.

A limited but significant retardation of growth of challenge bacilli was observed in both pulmonary and splenic tissues of successfully vaccinated animals. This retardation was manifested chiefly during the early stages of the infection. However, the numbers of virulent organisms recovered from the organs of either vaccinated or control animals were much the same at later stages of the infection.

The appearance of pulmonary immunity was related to the stage of development in vivo of the vaccinal population. Only after large numbers of vaccinal bacilli could be recovered from the tissues was heightened resistance observed.

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