1. Helenine prevents the establishment in mice of passive viral immunity by anti-Semliki serum of swine, rabbit, or guinea pig origin.

2. A period of 12 days must elapse, between the antiviral serum administration and challenge with virus, for prevention of the establishment of passive immunity to become apparent. This period is believed to correspond to that in which injected antibody persists in circulation in the injected host.

3. Helenine is effective in preventing the establishment of passive viral immunity by heterologous antiviral sera when it is administered any time during a period of 6 days, extending from 4 days before to 2 days after injection of the antiviral serum.

4. Helenine does not prevent the establishment of passive viral immunity by antiviral sera of mouse origin (homologous).

5. Evidence is presented to indicate that the phenomenon of the prevention of the establishment of passive viral immunity by heterologous antiviral sera is not effected directly, but rather is mediated through some substance that helenine induces the injected host to elaborate.

6. The capacity to prevent the establishment of passive viral immunity could not be exhausted by repeated preceding injections of helenine at 2- or 3-day intervals.

7. Evidence is presented to indicate that the helenine-induced material does not act upon antiviral antibody per se but rather on heterologous foreign protein that happens to be labeled as Semliki Forest virus antibody. This helenine-induced material, whatever its nature, appears to enhance the capacity of the injected host to recognize and dispose of foreign protein.

8. Statolon, a material that like helenine is a known inducer of interferon, is, like helenine, also capable of preventing the establishment of passive viral immunity by heterologous antiviral sera.

9. Experiments designed to determine whether the induced material responsible for the antipassive immunity effect of helenine is interferon have yielded inconclusive answers thus far.

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