Rabbits infected intravenously with virulent mammalian tubercle bacilli have in a majority of cases developed circulating antibodies to a slight but appreciable degree. The increase in titer was detected in one group of rabbits within the 2nd or 3rd week, in others during the 2nd month of infection.

Rabbits with residual pulmonary foci, resulting from infection 6 months or more previously with human tubercle bacilli, on reinfection with bovine tubercle bacilli promptly developed circulating antibodies strikingly in excess of those found during the course of primary infection. Such antibodies were present 6 or 7 days after reinfection.

The changes in titer during tuberculous infection as detected by the bacterial surface reactions and by phagocytosis were again, within the experimental error, in quantitative correspondence. The loss of phagocytosis-promoting power in heated normal serum involves an exception to this correspondence between surface and phagocytosis effects. This exception has already been discussed in an earlier paper (8).

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