By using a battery of poliovirus neutralizing monoclonal antibodies received from the Pasteur Institute, Paris, we carried out the study of the patterns of neutralization epitopes of more than 300 poliovirus strains isolated from cases of acute spinal paralysis and from healthy vaccinated children. We tried to find out the utility of this test for establishing the origin (wild or Sabin) of a poliovirus strain and the possible evolution of the pattern of neutralization epitopes. The study of the neutralization epitopes was carried out by neutralization index assay (NI). The neutralization index represents the difference between the titers of poliovirus strain determined in parallel in the absence, as well as in the presence of constant concentration (predetermined) for each monoclonal antibody. An epitope was considered to be present for a NI > or = 2 and absent for a NI < or = 1.5. For types 1 and 3 polioviruses, Sabin-like epitopes have never been found to be present simultaneously with wild type epitopes on the same virus particle. Therefore, examination of the epitope pattern of these strains is very useful for intratypic differentiation. On the other hand, in type 2 poliovirus strains, Sabin and wild type epitopes sometimes coexist, in various combinations, thus rendering the interpretation of the results rather difficult.