As part of a study into the molecular basis of attenuation and reversion to neurovirulence in the Sabin poliovirus vaccines, we have determined the complete nucleotide sequence of a cloned DNA copy of the genome of P3/Leon/37, the neurovirulent progenitor of the type 3 Sabin vaccine strain, P3/Leon 12a1b. Comparison of the sequence with that which we previously obtained for the vaccine strain [Stanway, G., Cann, A. J., Hauptmann, R., Hughes, P., Clarke, L. D., Mountford, R. C., Minor, P. D., Schild, G. C. & Almond, J. W. (1983) Nucleic Acids Res. 11, 5629-5643] indicates that attenuation has been brought about by a maximum of 10 point mutations, at least 5 of which are likely to be of minor significance. Predicted amino acid sequences of all the known virus-encoded proteins show that amino acid substitutions have occurred at only three positions. Two of these are in structural proteins (i.e., Ser----Phe in VP3 and Lys----Arg in VP1), and the third, Thr----Ala, is in the nonstructural protein P2-3b. The distribution and nature of nucleotide and amino acid sequence differences suggest that a single base substitution may be responsible for the attenuated phenotype of the vaccine strain.