Sixty-five premature infants were given CHAT type 1 live poliovirus vaccine. Seventeen other infants were retained as controls. Although 92% of the infants were infected with the attenuated poliovirus, only 59% of infected infants had a definite antibody response. The size of the infant did not seem to influence the duration of intestinal infection. In 86% of the infants there were significant levels of transplacentally acquired antibodies with an over-all half-life estimated at 23 days. Moderately high titers of homotypic maternal antibodies in the infants did not interfere with infection, nor was there obvious inhibition of active immunologic responses. Thirty-one of the infants were given a second feeding of the CHAT virus 2 to 5 months after the first administration. The primarily immunized infants were relatively, but seldom completely, resistant to re-infection. In contrast, infants that did not form antibodies after the primary infection seemed to be fully susceptible to re-infection. Moreover, there was no evidence of immunologic tolerance to poliovirus in these infants, for after the second infection poliomyelitis antibodies were invariably found. We concluded that, while premature infants are highly susceptible to infection with CHAT poliovirus, their immunologic responsiveness—just as in newborn full-term infants—is distinctly less than in infants 2 or 3 months old, but that exposure to poliovirus in early life does not impair later immunologic functioning.