A complete cDNA copy of the genome of the attenuated type 1 poliovirus vaccine (Sabin 1) strain was constructed and inserted into the EcoRI site of the plasmid pBR325. When cultured mammalian cells were transfected with this recombinant plasmid, 20 to 50 poliovirus plaques per 10 micrograms plasmid DNA were observed. Fingerprints of the RNA of the recovered virus showed no changes when compared with those of the parental virus genome, an observation indicating that the primary structure of the cloned cDNA is a reflection of authentic poliovirus RNA. The recovered virus had the same properties as those of the Sabin 1 strain in regard to antigenicity, sensitivity to temperature, and dependency on bicarbonate concentration. These results suggest that the virus obtained by DNA transfection is indistinguishable from the Sabin 1 strain. The recombinant plasmid could therefore be used as a stable repository of the virus and as inoculum for the oral polio live vaccine.