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A complement-fixing antigen has been developed, using as source of material CNS tissue from newborn mice infected with the newborn mouse-adapted strain of the Lansing type, MEF1 virus. With this antigen, specific reactions have been obtained with sera from mice, cotton rats, and monkeys immunized with the Lansing-type virus, and from monkeys and chimpanzees convalescent from infection with this virus. Twenty-one of 35 human sera obtained from individuals convalescent from poliomyelitis were positive and 6 of 22 from apparently normal persons having Lansing-neutralizing antibody, while this held true for only 1 of 19 from those having no Lansing-neutralizing antibody. The fact that positive results were found in sera from patients having an infection with poliomyelitis virus of the Brunhilde type and at the same time no Lansing-neutralizing antibody brings up the possibility of the existence of a cross-reaction in complement fixation between the two types.

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.