Clinical and serological responses of infants to primary immunization with diphtheria-tetanus-pertussis (DTP) vaccine containing components of acellular pertussis vaccine (DTP-AC) were compared in a double-blind study with responses of infants receiving whole-cell DTP vaccine (DTP-WC). Three doses of either DTP-AC containing lymphocytosis-promoting factor (LPF) and filamentous hemagglutinin (FHA) or DTP-WC vaccine were given to infants at two, four, and six months of age. Nineteen infants received DTP-AC vaccine, and 20 infants received DTP-WC vaccine. Significantly more infants who received DTP-WC vaccine manifested fever, swelling, and/or total reactions than did infants who received DTP-AC vaccine. Infants who received DTP-AC vaccine had comparable antibody titers to LPF and significantly higher titers to FHA after three immunizations, as compared with the infants who received DTP-WC vaccine (P less than or equal to .001). The DTP-WC vaccine stimulated higher pertussis agglutination titers (P = .04) than did DTP-AC. The DTP-AC vaccine was immunogenic and significantly less reactogenic in infants than was the currently used DTP-WC vaccine.