235 healthy 10-12 years old school children were randomly immunized with either a booster dose of diphtheria-tetanus-acellular pertussis (dTap) or diphtheria-tetanus (dT) vaccine. For this booster immunization designed for school children and adults, the quantities of Bordetella pertussis antigens in the dTap vaccine had been reduced to one third of those of the Infanrix vaccine (SmithKline Beecham) commonly used for infants. IgG antibodies and cell-mediated immune (CMI) responses to pertussis toxin (PT), pertactin (PRN) and filamentous hemagglutinin (FHA) were assessed by an enzyme immunosorbent assay and in vitro proliferation of peripheral blood mononuclear cells, respectively. Before immunization, 55%, 80% and 99% of children had detectable serum IgG antibodies to PT, PRN and FHA, whereas CMI response was found in 35%, 27% and 50% of children, respectively. After immunization, a 20-30-fold increase in geometric mean level (GML) of antibodies to the pertussis antigens occurred and CMI response to PT, PRN and FHA was seen in 88%, 94% and 100% of children, respectively. Adverse reactions following the immunization were rare. The results show that booster immunization with an acellular pertussis vaccine with reduced concentrations of antigens induces both antibody and CMI responses and support further studies of this pertussis vaccine in school children.