Antibodies protecting mice against 10-1000 lethal doses of Pneumococcus Type II were found in the fresh serum of 21 of 32 normal rabbits tested in this study. In general, the protective effect of the serum in mice was closely correlated with the bacteriostatic action of the heparinized blood and both sorts of reaction appeared to be type-specific; the best evidence being the absorption of the protective antibody by killed cells of Pneumococcus Type II, but not by R cells.

Partial or complete loss of the antibody during 3 weeks' storage at 2 to 5 C, its preservation for 3 months at −15 to −20 C, its fluctuating titre in the circulating blood of some animals and its persistence in one rabbit for as long as 9 months were observed.

These findings, associated with the fact that no normal rabbit whose serum failed to protect mice against 1 lethal dose of Pneumococcus Type II gained that capacity after injection of soluble antigen from Pneumococcus Type I (Dubos' soluble antigen), pose a question of the validity of a number of reports that a soluble pneumococcal antigen may elicit, in rabbits, the production of antibodies capable of protecting mice against heterologous types.

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