Because only primates are susceptible to polioviruses, the neurovirulent safety and consistency of oral poliovirus vaccine (OPV) were assayed in the monkey neurovirulence test. After the development of transgenic (Tg) mice carrying the gene for human poliovirus receptor (PVR), the suitability of these mice to replace monkeys for OPV testing was evaluated. Two lines of Tg mice, TgPVR1 and TgPVR21, were tested. The TgPVR21 mice, inoculated in the spinal cord, were as sensitive as monkeys in discriminating between type-3 and type-2 OPV lots that had passed and those that had failed the monkey neurovirulence test. Results of the new molecular assay by polymerase chain reaction and restriction enzyme cleavage indicated that each OPV lot contained minuscule amounts of neurovirulent revertants in the viral genome. All type-3 OPV lots that failed the monkey neurovirulence test had higher percentages of 472-C revertants than did lots that passed this test. Analysis of multiple type-3 OPV lots also indicated a good correlation between the contents of 472-C revertants and results of the TgPVR21 mouse test. An overview of a significant set of data suggests that the TgPVR21 mouse model is suitable for the evaluation of type-3 and type-2 OPV. The necessity of the TgPVR mouse test for the neurovirulence of type-1 OPV, which is the most stable of the three Sabin strains, is under consideration.