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1. Hydrochloric acid extracts of Streptococcus hæmolyticus contain type-specific, as well as non-type-specific, substances. The precipitates formed by these crude extracts with homologous antibacterial serum are flocculent, while those obtained with heterologous serum are usually disc-like. 2. The type-specific substance may be detected by the use of anti-bacterial sera absorbed with heterologous strains of hemolytic streptococci. Such absorbed sera are type-specific: they are precipitated only by extracts of strains of the homologous type. 3. Any heterologous strain of hemolytic streptococcus absorbs the antibodies for all other heterologous strains, but homologous strains absorb type-specific antibodies as well. Numerous repeated absorptions with heterologous hemolytic streptococci tend to lower the titer of the type-specific antibody. A possible explanation of this fact is suggested. 4. Three strains did not yield a type-specific substance; and it seems probable that they had lost this function because of long continued cultivation in artificial media. 5. Classification based on the precipitin test with absorbed serum agrees with that previously determined by agglutination and protection tests. The method is, therefore, applicable to the problem of classification of the hemolytic streptococci.

THE ANTIGENIC COMPLEX OF STREPTOCOCCUS HÆMOLYTICUS