• We administered diphtheria, tetanus, and pertussis (DTP) vaccine containing acellular (lymphocytosis promoting factor and filamentous hemagglutinin) pertussis vaccine to three groups of 20 children each (4 to 6 years, 17 to 21 months, and 5 to 9 months of age). All the children tolerated the vaccine well; no reactions occurred that contraindicated further Immunization. Older children had significantly more local (redness or swelling) and systemic (fever or fretfulness) reactions than younger children. Eighty percent to 90% of the children in the two older age groups had fourfold or greater increases in antibody titers to DTP antigens one month after vaccination. The postvaccine concentrations of antibody to tetanus and diphtheria were greater than 0.01 IU/mL In all children. Serologic responses to lymphocytosis promoting and filamentous hemagglutinin varied with age; significantly more older children than younger children had four-fold or greater increases. Acellular pertussis DTP vaccine was antigenic in young children and was less reactogenic than standard whole cell DTP vaccine according to rates reported in previous studies.