1. Virgin and pregnant Swiss mice are equally susceptible to intracerebral inoculation of St. Louis encephalitis virus.

2. Following subcutaneous vaccination with the St. Louis virus, the great majority of virgin Swiss mice become immune to subsequent intracerebral injection of 10,000 M.L.D. of the virus.

3. The majority of mice vaccinated during pregnancy do not become immune to even as little as 500 intracerebral M.L.D. of the virus. The depression of the ability to acquire immunity against the virus is most marked when the vaccination is carried out late in pregnancy, but it is also demonstrable when the mice are vaccinated early in the gestation period and during the first 2 weeks postpartum. At 7 weeks postpartum the response to vaccination is more nearly like that of virgin mice.

4. Pregnancy not only interferes with the development of acquired immunity but it also diminishes a previously established immunity.

5. Offspring of the mice vaccinated during pregnancy are not immune to 100 M.L.D. of virus.

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