By means of rapid serial passages, including 3 successive "blind" passages, the MEF1 strain, a Lansing-type poliomyelitis virus, has been adapted to new-born mice. The virus can readily be propagated in newborn mice, in which fully adapted virus induces in almost all inoculated animals the experimental disease, resulting in a much greater infectivity for the central nervous system and a uniformly short and regular incubation period.

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