The antigenic mutants, which were significantly different intra-typically from the parental Mahoney-1709 type 1 poliovirus, were repeatedly isolated after a number of serial virus passages in the presence of homologous anti-Mahoney sera. The levels of the antigenic variation of those mutant viruses varied, depending on the individual antiserum employed for mutant selection and the passage numbers with same antiserum. The susceptibility to certain serum inhibitors of the antigenic mutants varied

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... according to the antisera AMS 3 and AlVlS 4 employed for the mutant selection, but was not dependent on the passage numbers using the same antiserum. There is evidence that the antisera contained antibodies indistinguishable from the serum inhibitors in the specific antiviral activity, and that the production of these antibodies varied with the results of individual variations in the animals receiving the antigenic stimulus.