One of the problems raised by the use of the attenuated oral poliovirus vaccine (OPV) Sabin strains is the genetic instability of the attenuated phenotype upon multiplication in vivo. Nucleotide sites critical for attenuation were identified for each of the three poliovirus serotypes. One important position lies in the 5' non-coding region of the genome of each of the three OPV strains, at nucleotide 480 in type 1, 481 in type 2 and 472 in type 3. Point mutations at these positions were usually selected upon multiplication in vivo as substitutions of the vaccine-type residue. The reversion was found to correlate with an increased degree of neurovirulence. To screen easily for this mutation in a great number of strains, we developed a site-specific polymerase chain reaction method based on the property of the Taq polymerase to elongate only primers with a perfect homology at the 3' extremity. We screened for this mutation in five type 1 and nine type 2 polio vaccine-derived strains isolated from vaccine-associated paralytic poliomyelitis (VAPP) cases and in 16 such strains isolated from healthy vaccinees. All 14 strains isolated from VAPP presented the reversion. Of the eight pairs of type 1 isolates from healthy vaccinees, four presented the reversion 3 days after vaccine administration and all but one at 7 days postvaccination. These results support the involvement of the 5' non-coding specific nucleotide sites in the reversion to neurovirulence of attenuated polio vaccine strains upon multiplication in the human gut.(ABSTRACT TRUNCATED AT 250 WORDS)