We believe that our experiments have shown that the serum of rabbits and sheep immunized with cultures of Treponema pallidum acquires spirochæticidal properties for these culture spirochætes. The normal serum of these animals also possesses spirochæticidal action if used in sufficient quantities, and the action of the immune serum represents probably an increase of the antibodies normally present. Both normal and immune spirochæticidal properties are destroyed by heating to 56°C. The spirochæticidal action of the immune serum can be reactivated by the addition of fresh normal serum of the same species, insufficient in amount to exert a spirochæticidal effect by itself. The structure of these spirochæticidal bodies, therefore, is entirely analogous to that of the well known bactericidal antibodies known to exist in antibacterial sera. We do not wish to have these results interpreted as applying to virulent spirochætes as well as to culture spirochætes. A subsequent publication will demonstrate why we specify this at present.