Vaccination of susceptible mice with St. Louis encephalitis virus induced an acquired immunity to intracerebral inoculation which appeared within 1 week. When first demonstrable this immunity was at a very high level and it remained so for about 6 weeks, after which time it declined and disappeared completely between the 12th and 20th week after vaccination.

Humoral antibodies of significant titre could not be demonstrated during the first 6 weeks after vaccination when actual immunity to intracerebral inoculation was at its highest level. Antibodies were first detected 8 weeks after vaccination, when immunity was decreasing. The humoral antibody titre reached its peak at 23 weeks, at which time immunity had completely disappeared. A high level of circulating antibody was still present 37 weeks after vaccination.

The evidence presented leads to the conclusion that humoral antibodies do not play a major part in the immunity acquired by susceptible mice after vaccination against St. Louis encephalitis virus.

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