1. The murine virus may be grown in embryonic mouse brain-serum ultrafiltrate cultures. The virus fails to grow in embryonic chick tissue cultures or in fertilized egg preparations.

2. Some relationship can be demonstrated between the amount of nervous tissue and the infectivity of the culture.

3. Optimum titers of virus potency (10–6) can be obtained by adjusting the pH of the growing culture at 7.3 to 7.6.

4. A simple pH inactivation curve for virus alone and for virus when actively growing in tissue culture has been obtained.

5. The rate of virus propagation, as determined by potency tests in mice, has been established for cultures which were seeded with large or small amounts of virus. The murine virus "grows" relatively fast. The optimum titer for a large inoculum was reached in 19 hours, for a small inoculum in 72 hours.

6. With extended subcultivation in vitro the cultured virus shows a loss of infectivity for mice by peripheral injection. However, potency as determined by intracerebral injection remains constant.

7. Mice surviving inoculation of culture virus by routes other than intracerebral acquire a relative resistance to reinfection with mouse passage virus.

8. The murine culture virus passes without difficulty through collodion membranes with an A.P.D. of 30 mµ. Its particle size may therefore be estimated as lying between 10 to 15 mµ.

9. On a basis of in vitro activity and cross infection, the murine culture virus is distinct from the virus of lymphocytic choriomeningitis.

10. Murine culture virus may be used as an interfering agent to block infection with poliomyelitis virus in monkeys. The interaction between the two viruses seems to be quantitatively limited. Such interference, with the present potency of culture virus, operates effectively only against comparatively small doses of monkey virus.

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