The development of poliovirus neutralizing coproantibodies was studied in 14 newborn infants following a single immunization with either live oral trivalent or parenterally administered inactivated poliovirus vaccine. Poliovirus coproantibodies, predominantly directed against poliovirus type 2, appeared in stool specimens of all orally immunized infants within 2 to 3 weeks. Oral reimmunization of one infant 47 days after the primary dose resulted in the production of coproantibody to all three types of poliovirus. In contrast, parental immunization of five infants did not stimulate fecal antibody production. IgA was the predominant, and sometimes the only, immunoglobulin detected in fecal extracts. Although stools of newborn infants usually did not contain IgA, the infants secreted fecal IgA at the time of the appearance of polio coproantibodies. Furthermore, alimentary IgA was demonstrable in the control group of older infants, even in the absence of detectable serum IgA. Our results indicate that local intestinal antigenic stimulation is a necessary requirement for the production of polio antibodies by the secretory IgA immunoglobulin system of the alimentary tract. These findings are presented as a mechanism for the alimentary immunity to homotypic poliovirus reinfection which follows oral polio immunization of natural infection.