No indigenous cases of poliomyelitis have occurred in New South Wales since 1970, and it was presumed that herd immunity was high. However, our survey of Sydney's children by the neutralization test showed that only 42% of children had antibody levels (titre 8 or higher) to all three serotypes, 30% had antibodies to two, 16% to one, and 12% had no demonstrable antibodies at all. The immune status could not be determined with confidence from the immunization history, as 48% of the children who were reported as having received four or more doses of Sabin vaccine were not fully immune. When children who had inadequate levels of antibodies were given a single dose of Sabin vaccine, 77% of them achieved adequate antibody levels to all three viruses. The Health Commission of New South Wales subsequently undertook a mass Sabin immunization campaign of primary-school children, and reviewed the recommended immunization schedule.