The attenuated Type 3 strain of Oral Poliovirus Vaccine (OPV), Leon 12a1b, was derived by Albert Sabin by plaque purification of a progenitor wild-type strain Leon/37 after multiple rapid passages in cell culture at reduced temperature. Nucleotide sequences of both strains determined more than 10 years ago by sequencing cDNA clones differed at 11 nucleotides. Several manufacturers subsequently further plaque purified the Leon 12a1b strain and use re-derived strains to manufacture OPV. Because of the high heterogeneity of RNA viruses, clones derived from individual RNA molecules do not necessarily represent a true consensus sequence of the source strain. We determined nucleotide sequences of Leon/37 and sub-clones of the attenuated Leon 12a1b strain using a PCR procedure avoiding molecular cloning. Mutations unique for commercial substrains of the Sabin 3 virus were identified, as well as a number of additional differences between sequences of the wild-type and attenuated Leon strains, some of which may be related to attenuation of neurovirulence.