1. A substance extracted from group A hemolytic streptococcus is described, which induces active immunity in mice, and in rabbits gives rise to precipitins and to protective antibodies passively transferable to mice.
2. The active immunity in mice is principally type-specific, but some degree of non-type-specific immunity is also developed. The passively transferable protective antibodies are type-specific with only a slight suggestion of non-type specificity. In the precipitin test, the rabbit immune sera give both type-specific and non-type-specific reactions which have not been fully analyzed serologically.
3. Substances contained in the extract absorb the protective antibodies from the serum of rabbits immunized with whole hemolytic streptococci.
4. The most satisfactory method of extraction so far developed is fully described. Chemical tests on the material are consistent with the presence of protein and nucleic acid.
5. The type-specific M substance, prepared as previously described, was compared in some of its antigenic properties with the above mentioned substance. It was found capable of inducing active immunity in mice and of absorbing protective antibody from anti-bacterial immune serum in a manner qualitatively similar to that obtained with the preparations made by the newer methods.