The MS character of polio-virus particles is described. Strains that produced relatively large plaques, up to 6 mm. in diameter, on a monkey stable (MS) cell line, were classified as MS+. Such strains were found to include the highly virulent poliovirus strains. Attenuated strains appeared deficient in this gene, for they produced tiny plaques less than 1 mm. in size, or no visible plaques at all; they were considered as MS mutants.

Strains of maximum neurovirulence for the monkey possessed the MS+ and d+ characters, while those of greatest attenuation were MS d. Strains possessing the MS d+ character showed high or intermediate attenuation.

Virulent strains grew equally well in MS or primary monkey kidney (MK) cells. Attenuated strains gave lower titers in MS cultures than in MK cultures. While MS cells after infection with a virulent virus yielded about 100 to 200 PFU per cell, only one PFU per MS cell was detected after infection with an attenuated virus.

A study of newly isolated Type 3 strains showed them to consist of the MS+d+ or MS d+ type. The MS+d+ virus proved to be highly paralytogenic for monkeys even by the intramuscular route, while the MS d+ virus was of the partially attenuated type.

The study of in vitro characters of viruses from children fed attenuated poliovirus offers a possibility for following genetic changes of the viruses after multiplication in the human enteric tract.

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