Comparative tests of the monkey neurovirulence of the Lederle-Cox and the Sabin strains of poliovirus, which are candidates for an orally administered vaccine, have indicated that children have been fed poliovirus strains possessing greater neurotropic activity than had been suspected. There was also evidence that, in passing through human hosts, some of the vaccine strains underwent genetic changes with respect to at least two properties in tissue culture, namely, the capacity to grow at 40 C (T marker) and the capacity to grow at low bicarbonate concentration (d marker). These changes after human passage were associated with increases in monkey neurovirulence. A further study of 596 postvaccination fecal specimens revealed that marked, though not complete, interference occurred between the vaccine virus and the enteroviruses already circulating in the population. This interference affected adversely the formation of the desired antibodies. It has not been proved that the strains used as orally given vaccine are dangerous either for the vaccinated child or for the community, but caution is called for, especially in view of the genetic instability of the attenuated strains of poliovirus currently available.