Antigenic mutants of poliovirus (Sabin strain, serotype 1) were isolated by the resistance of the virus to anti-Sabin neutralizing monoclonal antibodies. The amino acid replacements within the capsid protein sequence causing the altered antigenicity were identified for each of 63 isolates. The mutations cluster into distinct nonoverlapping peptide segments that group into three general immunological phenotypes on the basis of cross-neutralization analyses with 15 neutralizing anti-Sabin monoclonal antibodies. Location of the mutated amino acid residues within the three-dimensional structure of the virion indicates that the majority of these amino acid residues are highly exposed and located within prominent structural features of the viral surface. Those mutated amino acid residues that are less accessible to antibody interaction are often involved in hydrogen bonds or salt bridges that would stabilize the local tertiary structure of the antigenic site. The interactions of the peptide segments that form these neutralizing sites suggest specific models for the generation of neutralization-resistant variants and for the interaction between the viral surface and antibody.