1. A neurotropic murine virus was isolated by passing poliomyelitis virus (SK strain) from the monkey to cotton rats and white mice.

2. The murine virus has been grown in tissue culture consisting of embryonic mouse brain in ox serum ultrafiltrate.

3. The symptoms and lesions produced by the murine infection compare in all respects with those of poliomyelitis in monkey and man.

4. The murine virus, while highly pathogenic for mice and cotton rats, is non-pathogenic for albino rats, guinea pigs, and rabbits. It possesses limited pathogenicity for rhesus monkeys.

5. Although producing no paralysis in the above mentioned refractory animals, the murine virus may be recovered in active form from neural and extraneural sites of infected albino rats, guinea pigs, and monkeys, but not from rabbits.

6. The identity of the murine and monkey virus is further suggested by cross-neutralization between the murine virus and homologous (SK) and related (Aycock) antipoliomyelitis sera, as well as between homologous and related monkey poliomyelitis virus and antimurine virus sera.

7. Immunization of monkeys with live murine virus, in the form of mouse brain or tissue culture, seems to confer some degree of resistance against subsequent infection with the homologous poliomyelitis monkey virus.

8. The presence of the murine virus in the central nervous system of infected monkeys appears to interfere with the propagation of SK and Aycock poliomyelitis monkey virus in the same animal.

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