An acellular pertussis-component diphtheria-tetanus-pertussis (AC-DTP) vaccine was compared with a currently licensed, whole-cell pertussis-component DTP (WC-DTP) vaccine for reactogenicity and immunogenicity when given as the fourth DTP immunization in sixty 18- to 24-month-old children. Reactions over the first 48 hours were significantly less common in the AC-DTP vaccine recipients, as follows (WC-DTP/AC-DTP): fever, 85%/5%; redness, 70%/12.5%; tenderness, 100%/22.5%; swelling, 35%/10%; fretfulness, 70%/12.5%; anorexia, 35%/2.5%; and vomiting, 10%/0%. Antibody responses to pertussis antigens (agglutinogens, lymphocytosis-promoting factor, and filamentous hemagglutinin), diphtheria toxoid, and tetanus toxoid in AC-DTP vaccine recipients were comparable with those in WC-DTP vaccine recipients. The AC-DTP vaccine evaluated in this trial seems to be as immunogenic as WC-DTP vaccine while being markedly less reactogenic.