Concerns about the safety of whole-cell pertussis vaccines prompted development of acellular vaccines that are less likely to provoke adverse events because they contain purified antigenic components of Bordetella pertussis. Two diphtheria and tetanus toxoids and acellular pertussis (DTaP) vaccines--ACEL-IMUNE and Tripedia--have been licensed for several years, but (until recently) only for administration of the fourth and fifth doses in the series to children aged 15 months-6 years who previously had received three or more doses of diphtheria and tetanus toxoids and whole-cell pertussis (DTP) vaccine. Published reports indicate that, when administered to infants aged 2, 4, and 6 months, acellular pertussis vaccines are effective in preventing pertussis disease and associated with fewer local, systemic, and certain more serious adverse events than whole-cell pertussis vaccines. On the basis of these data, the Food and Drug Administration (FDA) has licensed three DTaP vaccines for use among children aged 6 weeks-6 years. Tripedia is now licensed for the initial four doses, and ACEL-IMUNE for all five doses of the diphtheria, tetanus and pertussis vaccination series. A third DtaP vaccine (Infanix) was licensed in January 1997 for the initial four doses of the series. Tripedia, ACEL-IMUNE, and Infanix are now recommended for routine vaccination of infants and young children, although whole-cell pertussis vaccines remain acceptable alternatives. Tripedia, ACEL-IMUNE, and Infanix are recommended for all remaining doses in the schedule for children who have started the vaccination series with one, two, three, or four doses of whole-cell pertussis vaccines. In September 1996, FDA licensed the use of TriHIBit (ActHIB reconstituted with Tripedia) for the fourth dose in the series of vaccinations against diphtheria, tetanus, pertussis, and Haemophilus influenzae type b disease.