Considerable genetic and antigenic heterogeneity was detected among a collection of 17 poliovirus type 3 strains isolated between 1939 and 1958 in studies using monoclonal antibodies and by T1 oligonucleotide mapping. Heterogeneity was detected even amongst a collection of nine viruses designated Saukett and assumed to originate from the same prototype virus. The monoclonal antibodies were found to differ in their strain specificities for poliovirus type 3 strains in virus neutralization or single-radial-immunodiffusion tests. Relationships between strains detected in this way were in general consistent with those detected by oligonucleotide mapping. One of the monoclonal antibodies (NIBp 56) was able to distinguish between certain Saukett virus strains which differed by as little as a single specific oligonucleotide. The heterogeneity detected amongst Saukett viruses is of potential practical importance since these strains are used widely in the manufacture of inactivated poliovirus vaccine.