1. Broth-grown cultures, cultures from blood agar slants and culture filtrates (Berkefeld N or V) of H. Streptococcus scarlatinæ are without appreciable effect upon the rabbit, no matter how large the dose or by what route introduced.
2. The active toxic principle of H. Streptococcus scarlatinæ for rabbits is intimately associated with the protein of the bacterial cell, and is not given off in the artificial medium during the growth activity of the organism, indicating, therefore, its endotoxic character.
3. The endotoxin is readily obtained from the viable scarlatinal cultures through the medium of the peritoneal cavity of the rabbit immunized against the homologous strain (Pfeiffer phenomenon). The toxic substance thus obtained we have termed a lysate.
4. The rabbit is highly susceptible to the in vivo prepared lysate of Streptococcus scarlatinæ, at least from the cultures we have employed. The degree of the toxic effect upon the rabbit depends upon the size of the dose and the route through which it is introduced. The specific effects range from mild to severe and fatal forms of toxemia as indicated by high fever, leucocytosis, paralysis and acute hemorrhagic glomerular nephritis.
5. The experimentally induced nephritic lesions are analogous in kind and variety to those of acute scarlatinal nephritis in man, including the "epithelial crescent" formation, hyaline thrombi of glomerular capillaries, hemorrhage into capsular space and necrosis of capillary tufts.