1. The susceptibility of mice to the lethal effect of preparations containing the oxygen-labile hemolysin (streptolysin O) of group A hemolytic streptococci has been studied. Injection of a single sublethal dose of the streptococcal preparation causes the development of resistance to the effect of a lethal dose injected subsequently.
2. Resistance is demonstrable 3 to 6 hours after the injection of the streptococcal preparation, persists for approximately 30 hours, and then disappears.
3. Resistance induced by the streptococcal preparation, although relatively specific, is directed not only against the streptococcal preparation but also against saponin. Mice made refractory to the streptococcal preparation and to saponin exhibit normal susceptibility to a number of other toxic agents, with the possible exception of the alpha toxin of Cl. welchii.
4. Mice injected with a sublethal dose of saponin develop resistance to the effect of a lethal dose of either saponin or the streptococcal preparation.
5. Resistance depends upon processes distinct from those underlying classical antitoxic immunity.
