Abstract: The present invention provides a method of classifying an unclassified live poliovirus vaccine as having an acceptable or unacceptable level of neurovirulence, comprising, prior to vaccine administration, the steps of: a) selectively amplifying a region of an unclassified poliovirus vaccine genome containing a nucleotide position predictive for increased neurovirulence using selectively mismatched primers, whereby a restriction endonuclease site in the selectively amplified region is created by introducing a site-specific mutation into the amplified region; b) digesting an amount of the amplified region with a restriction endonuclease that specifically cleaves the amplified sequences in revertant viruses which contain a reversion at the nucleotide position predictive for increased neurovirulence; c) digesting an amount of the amplified region with a restriction endonuclease that specifically cleaves the amplified sequences in nonrevertant viruses which contain the nucleotide position predictive for increased