1. The serum of rabbits treated intravenously with neosalvarsan, and of syphilitic patients treated intravenously with salvarsan or neosalvarsan, has a definite spirochæticidal action upon Spirochœta duttoni.

2. Although this spirochæticidal action is exerted in vitro, it can be demonstrated only after the treated spirochætæ are injected into susceptible animals.

3. A curative action of the serum of neosalvarsan-treated rabbits is exercised on mice infected with Spirochœta duttoni.

4. The spirochæticidal action of the serum of salvarsan-treated rabbits and patients is markedly increased by heating at 56° C. for thirty minutes.

5. The increased spirochæticidal action produced by heating is due in part to the destruction of some inhibitory substance contained in normal serum and in part to a direct effect of the heat upon the serum and salvarsan mixture.

6. Cerebrospinal fluid does not contain the inhibitory substance present in normal unheated serum.

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