New-borns raise limited antibody responses to most T cell-dependent antigens but little is known about neonatal T lymphocyte responses to vaccines. In this study, we compared the immune response induced by the hepatitis B vaccine in new-borns and nai;ve adults. Infants produced markedly higher serum anti-hepatitis B surface (HBs) antibody titres than adults. This was not associated with greater HBs Ag-specific Th2 cytokine responses but with lower primary IFN-gamma responses. At 1 year, the infant memory response to HBs Ag was characterised by higher Th2 responses than those of adults. We conclude that neonatal antibody and T cell responses to hepatitis B vaccine differ from those induced in adults.