Mice vaccinated with yeast were protected against subsequent infection with Type II pneumococci, but not against Types I or III. While the protection was not universal, as high a percentage of mice acquired active anti-Type II immunity, as was obtained by vaccination of another group of mice with Type II pneumococci themselves. This specific protection of mice by active immunization with the yeast antigen, is probably due to the same immunological relationship responsible for the anti-Type II reactivity of the antisera of rabbits immunized with yeast antigen.

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