The safety and immunogenicity of a booster dose of a new acellular pertussis component diphtheria-tetanus toxoids-pertussis vaccine (DTaP) were compared with whole cell pertussis component diphtheria-tetanus toxoids-pertussis vaccine (DTwP). Fifty children ages 15 to 18 months and 50 children ages 4 to 6 years were studied. The incidence of adverse reactions observed during the first 72 hours after vaccination in the DTaP/DTwP vaccinees were: pain, 32%/92% (P < 0.001); redness, 14%/24% (P = 0.2); swelling, 2%/14% (P < 0.03); fever, 52%/90% (P < 0.001); drowsiness, 14%/34% (P < 0.05); fussiness, 32%/88% (P < 0.001); and unusually poor appetite, 6%/42% (P < 0.001). The geometric mean titers of anti-pertussis toxin and anti-filamentous hemagglutinin antibody were also significantly (P < 0.001) higher in the DTaP compared to the DTwP recipients. When administered as a booster dose this DTaP vaccine, which has been chosen by the NIH for the second pertussis vaccine clinical efficacy trial, was more immunogenic for pertussis toxin and filamentous hemagglutinin and caused fewer and less severe adverse reactions compared with the Connaught DTwP vaccine used in this study.