Cellular immunity to vaccines is highly variable during infancy. This study addressed the hypothesis that these responses are governed by the pace of maturational changes in adaptive immune competence, in particular, cellular functions that underlie the postnatal transition from Th2 to Th1 "bias." Tetanus-specific cytokine responses were tracked in peripheral blood mononuclear cells collected from infants at months 2, 4, 6, 12, and 18. These were compared with polyclonal responses. Results show that the Th2 component of the vaccine response develops rapidly and remains stable, unlike interferon (IFN)-gamma production, which also is initiated early but commonly declines after the final priming dose at 6 months. However, between 12 and 18 months, the IFN-gamma component of the vaccine-specific response has a spontaneous resurgence that coincides with a parallel increase in overall IFN-gamma production capacity. The Th2 component of vaccine-specific responses was more prominent in children with atopic family history.