1. Sera from horses and rabbits under immunization with pneumococci of type I or II were titrated for agglutinative, precipitative, and complement-fixing activity with pneumococcal antigens and with agar. The type-specific reaction in type-I and -II antipneumococcal horse- and rabbit-sera developed earlier than or at the same time as nontype-specific or species activity. Comparable titers were usually reached earlier with immune rabbit- than with immune horse-sera.

  2. Pneumococcal nucleoprotein was precipitated before there was any evidence of cross-agglutination; this precipitative activity increased markedly in the serum of horses after the injection of live cells.

  3. Cross-agglutination occurred in those later bleedings which precipitated the heterologous type-specific carbohydrate and species-carbohydrate.

  4. Precipitation of hydrolyzed agar appeared early and did not seem to parallel cross-agglutination.

  5. Immune horse and rabbit sera differed sharply in their complement-fixation reactions. Rabbit sera fixed complement actively with cell-suspensions and pneumococcal nucleoprotein throughout the course of immunization; later bleedings from horses failed to give fixation with these antigens. Both continued to react with the lipid fraction of the pneumococcus throughout the immunization and for some time afterward. As has been previously reported by others, the rabbit-sera fixed complement with the specific polysaccharide; the horse-sera did not.

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