1. Hemolytic streptococci belonging to Lancefield's group A contain nucleoproteins closely related to those of pneumococcus.

  2. Sera against hemolytic streptococci of Lancefield's group B obtained by immunization of rabbits with sonically disintegrated bacterial cells showed strong serological reactivity with group A neutral sonic filtrate. This reaction is not reciprocal. In view of these facts it may be concluded that hemolytic streptococci belonging to Lancefield's group A are more closely related to pneumococci than to hemolytic streptococci belonging to Lancefield's group B in respect to the nucleoprotein components of these organisms.

  3. Sera obtained by injection of rabbits with sonically disintegrated group B organisms showed stronger cross-reactivity than rabbit sera obtained by immunization with intact organisms.

  4. The two strains of Streptococcus viridans used in this study seem to belong to an entirely different group as far as the nucleoprotein components are concerned.

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This study has been supported in part by a grant from The Commonwealth Fund.

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