1. Helenine injected intraperitoneally 24 hr prior to a regularly fatal dose of Semliki Forest virus saves most of the mice to which it is administered.

2. Mice saved by helenine develop no viral immunity and regularly succumb when rechallenged 2 wk later with the same dose of virus from which they were originally saved.

3. The time during which helenine is optimally effective in protecting mice from death by Semliki Forest virus covers a period of approximately 36 hr beginning after about 12 hr and extending to 48 hr before virus infection. When periods of less than 12 hr, or more than 48 hr, elapse between the time of helenine administration and virus inoculation, its protective effectiveness diminishes progressively.

4. Repeated injections of helenine at 2- or 3-day intervals, if continued long enough, exhaust the capacity of a host to respond favorably to helenine administered 24 hr before virus inoculation.

5. Helenine injections at intervals of 4, 3, and 2 wk before its administration 24 hr prior to infection do not decrease the effectiveness of this final dose in protecting mice from fatal infection by the virus.

The experimental results here reported indicate that, as suggested by the findings of earlier work, helenine does not act directly as an antiviral substance, but instead exerts its effect through some substance that it induces the host to elaborate. The nature of this induced antiviral substance is as yet unknown though, to judge from the failure of spared mice to acquire viral immunity, it appears to act at a stage in viral replication prior to that at which antigenic viral protein is produced.

The findings with helenine and those thus far reported for interferon afford no factual basis for judging the relationship of the two, if any.

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