Pneumococci killed by acetic acid at pH 4.2, then allowed to become Gram-negative at pH 7.0, under conditions such that no cellular disintegration takes place, release in solution small amounts of a substance which is precipitable by acetic acid, and soluble at neutral reaction.

This soluble fraction injected into rabbits by the intravenous route causes the production of antibodies which afford definite protection to mice infected with virulent pneumococci of Types I, II, and III. Other types were not tried.

White mice immunized with this soluble antigen exhibit some active immunity to virulent pneumococci, but the results have been very irregular so far.

Soluble fractions, similar in properties and with apparently the same immunizing action, have been obtained from both virulent (S) and avirulent (R) cells of pneumococci.

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