1. The thermal death time of Trefonema pallidumin extracts from lesions in rabbits' testes was determined in vitro at fever temperatures using the Zinsser-Hopkins and Nichols strains.

2. The criteria to determine the persistence of infectivity of the heated extract were the following: the development of lesions on inoculation into rabbits, dark-field examination of tissue from the lesions, the outcome of blood Wassermann tests, and of reinoculation tests.

3. The thermal death time of the two strains of spirochetes was approximately the same, although the Nichols strain was somewhat the more resistant. In the case of the latter 5 hours at 39°C., 3 hours at 40°C., 2 hours at 41°C., and 1 hour at 41.5°C., were required to render infective extracts innocuous to other rabbits.

4. The thermal death time of T. pallidum in testicular extracts in vitro at fever temperatures is so short as to suggest that induced fever may be useful therapeutically in human syphilis.

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