Responses to the oral administration of Sabin's attenuated type 1 poliovirus vaccine were studied in a closed, institutional community under conditions that made it possible to compare individuals whose immunity had been acquired naturally with those who had received the Salk formalinized vaccine. The results showed that all infections induced by the vaccine were symptomless and that they spread readily through the small institutional community. The children whose antibodies had been acquired from the Salk vaccine were easily infected with the attenuated virus, whether they received it by mouth or through contact with individuals infected in the same cottage. In contrast, children whose antibodies had been acquired by previous natural infection were more resistant, but this depended somewhat on the dose of vaccine given. There were indications that passage of the virus through certain individuals slightly increased its neurotropism for the monkey.