Monoclonal antibodies to poliovirus type 3 secreted by nine mouse hybridoma cell lines were tested for virus-strain specificity with a collection of wild and vaccine-like type 3 viruses, the genetic characteristics of which had been determined by T1-oligonucleotide mapping. Four of the monoclonal antibodies had both virus-neutralising and antigen-blocking activity, and five had antigen-blocking activity only. Of the four virus-neutralising antibodies, two (NIBp138, NIBp165) reacted only with Sabin vaccine virus and other poliovirus type 3 strains with Sabin-like T1-oligonucleotide maps. Two antibodies (NIBp132, NIBp134) neutralised a broad range of type 3 viruses. Of five monoclonal antibodies with antigen-blocking activity only, four (NIBp135, NIBp136, NIBp140, and NIBp141) reacted exclusively with Sabin virus and strains with Sabin-like maps, and one (NIBp139) reacted with all type 3 strains tested. The vaccine-specific monoclonal antibodies, particularly NIBp138 and NIBp165, have considerable potential value in distinguishing wild type 3 viruses from those of Sabin vaccine origin. This conclusion was strongly supported by the finding that certain viruses which showed genetic evidence of being related to Sabin virus reacted well with vaccine-specific monoclonal antibodies despite having undergone genetic drift away from vaccine virus as indicated by changes in up to 30% of their oligonucleotides.