Evidence is presented that the serum of mice paralyzed with the Lansing strain of poliomyelitis virus possesses the capacity to protect normal mice against the same virus when mixtures of virus and serum are inoculated intracerebrally.

The virus-neutralizing action was present mainly in mice paralyzed between the 3rd and 6th days following inoculation and was rarely demonstrable in the serum of mice developing paralysis more than 10 days after inoculation.

Serum from non-paralyzed mice had approximately the same neutralizing capacity when obtained during the same period following inoculation in which serum from paralyzed mice showed the greatest effect.

Neutralization of the virus was not obtained with serum from normal mice or from mice inoculated with normal nervous tissue.

Further investigation of the phenomenon is in progress.

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