An experimental study of three strains of Russian spring-summer encephalitis virus and one of louping ill virus has yielded the following results:—

1. The sera of mice hyperimmunized to the viruses of Russian encephalitis and louping ill respectively have produced complement fixation with both antigens in almost precisely the same titer.

2. In neutralization tests hyperimmune sera against the Russian virus strains protected against louping ill virus to the same extent as against the Russian virus strains. Conversely, hyperimmune sera against the louping ill virus protected against the Russian viruses, although to a less degree than against louping ill virus.

3. In cross-resistance tests in mice, a vaccine consisting of formolized Russian virus gave strong protection against this latter and moderate protection against louping ill virus. Formolized louping ill virus gave moderate protection against infection with louping ill and considerably less against the Russian virus.

4. Serum from an individual recovered from a laboratory infection with louping ill virus contracted in 1933 gave positive complement fixation and neutralization tests with the Russian spring-summer encephalitis, as well as with louping ill virus.

5. Serum from a patient who became infected with either Russian or louping ill virus or both while working with the viruses in the laboratory in the fall of 1942, gave positive reactions on complement fixation and neutralization tests against them both.

6. No such similarities have been found with other central nervous system viruses. Hence it would appear that they are specific.

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