1. The invasion and multiplication of Bacterium tularense has been followed quantitatively after inoculation of the virulent organisms in normal, vaccinated and recovered white rats.

  2. Virulent Bact. tularense spread by the usual lymphogenic and hematogenic routes in normal, vaccinated or recovered white rats.

  3. Rats immunized by vaccination with killed cultures or by recovery from a previous sublethal infection are able to limit the net increase of virulent organisms and to sterilize their tissues.

  4. The routes of spread and the increase of organisms in immune animals differ only in a quantitative fashion from those observed in normal animals.

  5. Recovered rats are more solidly immune than vaccinated animals, as indicated by the smaller number of organisms recovered from the tissues and by their lower mortality rate after multiple lethal doses of virulent organisms.

  6. The behavior of avirulent strains in normal, vaccinated and recovered animals varies in degree but not in kind from virulent strains.

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