Horses immunized to Type I pneumococci developed serum, 0.1 cc. of which protected against 0.5 cc. of a virulent culture, 0.000001 cc. of which killed mice in less than 40 hours. Protective tests of serum from horses immunized to Type II organisms varied, 0.1 cc. protecting, however, in certain instances against 0.1 and 0.01 cc. of virulent homologous culture. Types I and II sera obtained in our experiments with culture sediment and whole culture did not vary markedly for a given type and corresponded closely in their protective titer with samples of sera received from The Rockefeller Institute Hospital. It is therefore evident that the following minimum standard of 0.1 cc. of serum to protect mice against at least 0.2 cc. of virulent cultures can and should be maintained when serum is to be used for the treatment of cases. By further study and comparison of these different methods of immunization it is hoped that sera of greater potency may be produced, but as yet this has only been accomplished in exceptional instances.

A horse immunized with Type III (Pneumococcus mucosus) developed serum having a slight degree of protection for mice against the corresponding organisms. This serum was sufficiently potent, however, to cause prompt and complete agglutination when combined with fresh untreated homologous organisms, thus avoiding the preliminary treatment to remove the capsule which has previously been held necessary. As a diagnostic aid in the differentiation of pneumococcus strains, the serum has proved of distinct value.

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