Oral Poliovirus Vaccine (OPV) developed by Albert Sabin consists of live attenuated polioviruses of three serotypes which replicate in the intestinal tracts of vaccine recipients and elicit life-long protective immunity. Similar to other RNA viruses, OPV is genetically unstable and undergoes quantitative changes in its attenuation markers (reversion) during replication in vaccinees and in cell cultures during vaccine production. Therefore every batch of OPV must be rigorously tested in monkeys to ensure genetic consistency and safety of the final product. A total of almost 200 monkeys is required for control of each batch of trivalent OPV. Almost 40 years of OPV production world-wide showed that OPV of types 1 and 2 rarely, if ever, fail the monkey neurovirulence test (MNVT), while it is not uncommon for type 3 OPV to have neurovirulence in excess of the acceptable level determined by testing a reference vaccine lot. Recent progress in understanding the molecular basis of OPV neurovirulence, attenuation, and reversion revealed several point mutations responsible for these properties. Our laboratory at FDA has developed a new molecular approach for direct quantitation of neurovirulent mutations in batches of OPV. The method, called Mutant Analysis by PCR and Restriction Enzyme Cleavage (MAPREC), demonstrated that all batches of type 3 OPV contain measurable amounts of C at nucleotide 472 of the viral genome (instead of U in the Sabin strain), and that the content of 472-C in batches failing the MNVT was consistently elevated. Quantitation of 472-C revertants was shown to be highly predictive for the outcome of the MNVT and is considered as a potential replacement for that test. The WHO is currently conducting an International Collaborative Study on MAPREC. Results of first phases of the Study found MAPREC to be a simple, reliable, and inexpensive molecular procedure that can successfully predict failure of type 3 OPV batches in the MNVT. This new molecular approach for monitoring genetic consistency of OPV substantially reduces the cost of its safety testing and might become a paradigm for control of other live viral vaccines.