During the summer of 1988, an outbreak of poliomyelitis caused by poliovirus 1 occurred in Israel, during which a national mass immunization campaign with oral poliovirus was undertaken. This prospective study was undertaken to assess the effect of maternal oral poliovirus immunization during the third trimester of pregnancy on neonatal immunity against poliovirus. Cord blood specimens of 88 neonates, born 2 to 7 weeks after maternal immunization, were examined for antipoliovirus antibodies and compared with 100 samples obtained from neonates 7 months before the outbreak. Blood samples were also obtained from the 62 mothers of neonates who had been immunized 2 to 5 weeks before delivery. Sera were tested for neutralizing antibodies to the 3 poliovirus types using a microneu-tralization technique. The geometric mean titer to poliovirus type 1 was significantly higher in neonates whose mothers were immunized during pregnancy (87.1) than in the offspring of the nonvaccinated group (53.0),P< 0.05. Two to 3 weeks after immunization, geometric mean titers against all 3 poliovirus types were higher in maternal blood than in cord blood whereas 4 to 5 weeks after vaccination a significant difference was found for type 3 only. Although oral poliovirus immunization during pregnancy resulted in higher neonatal antibody titers to poliovirus type 1, the proportion of newborns with titers of <1:8 to the 3 poliovirus types did not change significantly.