United States children two months of age were randomly assigned to two groups that received either the commercially availableoral trivalent poliovirus vaccine (OPV) or an injectable (inactivated) trivalent poliovirusvaccine (IPV)with a confirmed minimum Dantigen content of 27, 3.5, and 29 units for poliovirus types 1, 2, and 3, respectively. Vaccine was givenat two, four, and 18 months of age. Sera obtained from 439 children at two, four, and six months of age and from 85 children at 18 and 20 months of age were examined for neutralizing antibodies. The percentage of children with detectable antibodies and the reciprocalgeometricmean titers were similar for both groups at two months of age for antibodies to all three poliovirus types. At 20 months of age, all children but one had detectable antibodies to all three poliovirustypes. Significantly higher geometricmean titers against types 2 and 3 were noted at 20 months of age for the IPV group.