Many live virus vaccines derived by empirical routes exhibit temperature-sensitive (ts) phenotypes. The live virus vaccines that have been outstandingly successful in controlling poliomyelitis are the prime example of this phenomenon. The three live attenuated strains developed by Albert Sabin were derived from wild-type isolates by rapid sequential passage at high multiplicity of infection (MOI) in monkey tissue in vitro and in vivo, a regimen that yielded variants of reduced neurovirulence. Concomitantly, the three vaccine strains developed ts characteristics, a phenotype that correlated well with loss of neurovirulence. The reproductive capacity at supraoptimal (40°C) temperature, the rct phenotype, proved to be a useful property for monitoring the genetic stability of the attenuated virus during propagation, vaccine production, and replication in vaccinees. Nucleotide sequencing of the genome of the poliovirus type 3 attenuated virus and its neurovirulent wild-type progenitor (the Leon strain), revealed that only ten nucleotide changes, producing three amino acid substitutions, differentiated the attenuated derivative from its virulent parent despite its lengthy propagation in cultured cells. One of the three coding changes, a serine-to-phenylalanine substitution at position 2034 in the region encoding VP3, conferred the ts phenotype. A combination of nucleotide sequencing of virus recovered from a vaccine-associated case of paralysis and assay in primates of the neurovirulence of recombinant viruses prepared from infectious cDNA established that two of the ten mutations in the type three vaccine strain were associated with the loss of neurovirulence.