The immaturity of the neonatal immune system is associated with an increased susceptibility to infections. Studies in mice indicate that neonatal immune responses are biased towards the T helper 2 type, but little is known about helper T cell responses in human newborns. In this study, the oral polio vaccine was used as a model of early immunization to investigate the capacity of young infants to develop cellular immune responses. We show that neonatal immunization with oral polio vaccine induces the production of high titres of neutralizing antibodies but reduced proliferative and IFNgamma responses to polio antigens compared to immune adults. These data suggest that specific strategies will be required to immunize newborns against pathogens controlled by Th1 type immune responses.