The principal points brought out in the present article are the following.

1. Wild rats captured in this country carry in their kidneys a spirochete which possesses the morphological and pathogenic properties characteristic of Spirochata icterohamorrhagia discovered by Inada in the Japanese form of infectious jaundice.

2. Cultures of the American, Belgian, and Japanese strains of the spirochete were obtained by a special technique described, the first two strains having been cultivated artificially for the first time.

3. Animals actively immunized against the Japanese strain resist inoculation, not only of the same strain, but also of the Belgian and American strains. The Belgian strain produces immunity equally effective against all three strains. Experiments to. ascertain whether the immunity afforded by the American strain also protects against the Japanese and Belgian strains are in progress.

4. These findings warrant the conclusion that the spirochetes designated here as the Japanese, Belgian, and American strains are probably identical.

5. On account of its distinctive features, a new genus, Leptospira, has been suggested as the designation of this organism.

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