Irreparable damage to the anterior horn cells of the cervical and thoracic cord was found in a 20-week-old fetus whose mother was immune to poliomyelitis before conceiving but who was inadvertently given oral polio vaccine at 18 weeks gestation. Polio neutralizing antibody titres in sera, taken before and after pregnancy, were identical and were at levels normally regarded as providing protection. Unsuccessful attempts were made to isolate poliovirus from extracts of fetal brain, lung, liver and placenta. Fluorescent antibody tests were performed on various levels of the central nervous system and on the left and right extensor forearm muscles. Specific positive fluorescence to poliovirus 2 and 3 antigens was detected at dorsal spinal cord level only. One positive result was seen with Coxsackie A9 antiserum and fresh guinea-pig complement in the inflammatory cells in the right extensor forearm muscles.This experience, as yet unexplained, underlines the importance of ensuring that women are not pregnant prior to oral polio vaccination.