This study was designed to investigate the placental transfer of maternal poliovirus antibodies in full-term and pre-term infants. Two hundred healthy, Israeli born mothers and their infants, were enrolled immediately after birth. The study population comprised two groups: a full-term group of 150 mothers and their infants, and a pre-term group of 50 mothers and their infants (gestational age < 35 weeks). Maternal and umbilical cord blood samples were taken in all cases. Antibody titers against the three poliovirus serotypes and a polio virus type 1 strain that caused an outbreak in 1988 (epidemic strain 1) were measured by a microneutralization system. The proportion of individuals with protective titers against each of the poliovirus types tested was slightly lower in the infants compared with their mothers. When protection to all strains combined was tested, the difference between mothers and infants was significant (P < 0.05). Transplacental transfer to epidemic strain 1 was less effective--12% of the premature infants were not protected against it at birth. The geometric mean titers against poliovirus types 1, 3 and epidemic type 1 strain were significantly lower in the pre-term group than in the full-term group. In both the full-term and pre-term groups there were significant linear correlations between the maternal and neonatal antibody titers for each of the polio viruses tested. For all poliovirus types, the transfer of maternal antibodies to the full-term infant was significantly higher than the transfer of maternal antibodies to the pre-term infant (P < 0.001). Owing to diminished transfer of maternal antibodies, pre-term infants are at greater risk of poliovirus infection.