An outbreak of poliomyelitis in Finland resulted in the widespread circulation of wild-type 3 poliovirus strains that had antigenic properties distinct from the strains used to produce the attenuated and inactivated vaccines. Considerable variation was observed in the ability of broadly reacting monoclonal antibodies directed against type 3 poliovirus to neutralize the 54 strains examined. Sequential isolates from several persons showed an antigenic drift with these monoclonal antibodies and selected human sera. In addition, some faecal specimens were found to contain more than one antigenic variant. Primer extension sequencing of genomic RNAs of three plaque-purified antigenic variants isolated from one patient showed base substitutions in the region coding for the major antigenic site of poliovirus type 3. The resulting difference in the amino acid sequence in the virion protein VP1 could explain the differences observed in the neutralization of these strains by the monoclonal antibodies. Whether the observed changes in the antigenic characteristics of the sequential isolates represent true antigenic drift under immunological pressure or whether the emergence of the new variants is based on other modes of selection during replication is not known.