1. Addition of salvarsan to serum in vitro produces a spirocheticidal mixture which is increased in potency by heating.
2. The heated serum of salvarsan-treated patients is more spirocheticidal if it has been in contact with the clot over night than if it has been separated immediately after coagulation. This is not true with the serum from blood which has been salvarsanized in vitro.
3. The addition of salvarsan directly to serum produces a more potent mixture than results from the serum from blood to which salvarsan has been added in equivalent amounts.
4. The increase in activity of salvarsanized serum produced by heating to 56°C. is due in part to the removal of inhibitory substances in the serum and in part to a direct increase in spirocheticidal power in the heated salvarsan.
5. Both salvarsanized and neosalvarsanized serum are rendered more spirocheticidal by heating.
6. A more active spirocheticidal mixture is produced by mixing small amounts of salvarsan with the serum of a salvarsan-treated patient than by mixing the same amount with normal serum.