A new character, the A marker, for polioviruses is described. In the presence of Al+++ (1 to 100 mM) attenuated, but not virulent, strains of type 1 and type 2 polioviruses are stabilized so that they resist thermal inactivation at 50°C. Differences between the attenuated and virulent type 3 strains studied were of the opposite character and less marked.

By the use of optimal concentrations of Al+++ (12 mM for type 1 and 1 mM for type 2) attenuated strains can be separated from mixtures containing virulent type 1 or 2 strains.

The A marker was found to be correlated with the d but not the rct/40 or T marker. The A marker has been used for following genetic changes of vaccine strains after their multiplication in man.

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