Sixty-five per cent of unvaccinated pregnant women lacked antibodies to one or more poliovirus types, according to neutralization tests on serum done with HeLa cell cultures. Two injections of formalinized poliomyelitis vaccine given during pregnancy reduced the incidence of incompletely protected women to 18%. The infant at birth usually showed passively acquired antibodies at a level equal to or slightly below that of his mother, eliminated half of the antibodies in about five weeks after birth, and retained measurable amounts of antibodies for postnatal periods proportional to the content received at birth. On immunization with two injections of vaccine when the infants were one year old or less, their antibody responses were poorer than those of their mothers.