Using a set of neutralizing monoclonal antibodies targeted against the four known antigenic sites of poliovirus type 1, it was shown that three out of four antigenic sites are already present on 14 S subunits (pentamers of the structural unit). Site 3B, in contrast, is formed upon assembly of 14 S subunits into capsids. The data support the hypothesis that site 3B spans the boundary between pentamers in the virion.