Four vaccines containing diphtheria and tetanus toxoids and acellular pertussis vaccine (DTaP) are currently licensed in the United States for use among infants and young children. As of October 2000, two products, ACEL-IMUNE® (a product of Lederle Laboratories) and Tripedia® (Aventis Pasteur, Inc.) were licensed for the five-dose DTaP vaccination series. Two other vaccines, Infanrix® (SmithKline Beecham Biologicals) and Certiva™ (North American Vaccine, Inc.) are licensed for the first four doses of the vaccination series, beginning with the primary series at ages 2, 4, and 6 months, and for completing the DTaP series among children who began the series with diphtheria and tetanus toxoids and whole-cell pertussis vaccine. This report supplements the statement from CDC's Advisory Committee on Immunization Practices regarding use of acellular pertussis vaccines and summarizes data regarding reactogenicity of acellular pertussis vaccines when administered as the fourth and fifth consecutive doses. Increases in the frequency and magnitude of local reactions at the injection site with increasing dose number have occurred for all currently licensed DTaP vaccines. Extensive swelling of the injected limb, sometimes involving the entire thigh or upper arm, after receipt of the fourth and fifth doses of DTaP vaccines has been demonstrated for multiple products from different manufacturers. Because data are insufficient regarding the safety, immunogenicity, and efficacy of using DTaP vaccines from different manufacturers in a mixed sequence, ACIP continues to recommend that, whenever feasible, the same brand of DTaP vaccine be used for all doses in the vaccination series. When the vaccine provider does not know or does not have available the type of DTaP vaccine previously administered, any of the licensed DTaP vaccines can be used to complete the vaccine series.