The serologic response of school children receiving the second inoculation of poliomyelitis vaccine 5 months after the first injection was studied. Only 33% had antibodies against all three types of poliomyelitis virus immediately prior to the second inoculation. Two weeks following the delayed, second injection, 85% had antibodies against the three types of virus. The antibody titers attained by these children were at least as good as, if not better than, those of other age groups following two injections at an interval of 1 month.