I. Virus may be detected in the peripheral blood of horses during the febrile reaction following the injection of mouse neurotropic virus.
2. The more severe the reaction the more likely is free virus to be encountered.
3. This circulating virus retains its neurotropic character and after at least one generation in equines retains its attenuation.
4. Intracerebral injection of horses with mouse brain adapted virus after 158 passages did not produce a specific encephalitis but did produce a normal mild reaction followed by the development of a solid immunity.
5. The significance of these findings is discussed.