1. The chemical and immunological characteristics of the species-specific substance (C) of Streptococcus hæmolyticus are considered.
(a) It seems to be a carbohydrate because considerably purified preparations of C resisted prolonged tryptic and peptic digestion and were negative for the ordinary protein color tests but gave positive Molisch reactions to the limit of the precipitin titer. One such "purified" lot, however, had 4.2 per cent nitrogen and only 28 per cent reducing sugars on hydrolysis. Whether the nitrogen was due to impurities or was combined in the C substance itself, as is true of the Type I pneumococcus specific polysaccharide, cannot be stated without more material.
(b) The C substance forms precipitates with antibacterial sera prepared against heterologous, as well as against homologous hemolytic streptococci. These precipitates are typical discs like those formed by type-specific carbohydrates of other species of bacteria. C does not precipitate antinucleoprotein sera.
(c) While there is only slight direct evidence that the C substance is not antigenic, there is considerable indirect proof that this is the case. It probably is a haptene in the sense of Landsteiner.
2. A discussion is included of the chemical and immunological relationships of all the serologically active substances so far identified in extracts of the hemolytic streptococcus.