United States children two months of age were randomly assigned to two groups that received either the commercially available oral trivalent poliovirus vaccine ( OPV ) or an injectable (inactivated) trivalent poliovirus vaccine (IPV) with a confirmed minimum D-antigen content of 27, 3.5, and 29 units for poliovirus types 1, 2, and 3, respectively. Vaccine was given at 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 reciprocal geometric mean 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 poliovirus types. Significantly higher geometric mean titers against types 2 and 3 were noted at 20 months of age for the IPV group.