Four of five individuals possessing homotypic antibody in titers of 8 to 64 were infected on being fed Type III (Leon KP-34) poliovirus attenuated by Sabin by passage through tissue culture. None of the infected subjects or controls showed any evidence of illness which could be attributed to virus infection. There was no evidence of spread of infection to any of the control adult wardmates of the experimental subjects, although the two groups were in close contact: none of the controls excreted virus, none showed any antibody shift. One control who had no Type III antibodies at the start of the experiment was still antibody-negative on the 63rd day of the experiment. Three of the four individuals who became infected had naturally acquired-Type III antibodies; the other had antibodies induced by formalinized vaccine. Virus excretion in the stool was of short duration (7 to 13 days) in the three with natural antibodies, and lasted at least 6 weeks after feeding in the vaccinated child. Virus in the throat was detected only in the two persons receiving the larger virus dose (10(7.5) TCD(50)). In them it was present in small amounts between the 2nd and 6th day after feeding. No virus was detected in the blood of any of the infected individuals. The antibody responses of the four infected individuals were variable. There was no clear correlation with virus dosage, amount of virus excretion in the stools, or presence of virus in the throat. Only the child whose neutralizing antibodies were "Salk" vaccine induced showed a marked CF response. The virus excreted by two of the individuals who became infected, as tested in the 2nd tissue culture passage by monkey inoculation, was slightly more neurotropic than the virus which was ingested. Virus excreted by one of these individuals behaved as a virulent strain when tested by the in vitro plaque virulence test, while that isolated from the other had the characteristics of an attenuated strain in this test.