Purified acellular pertussis vaccine (12.5 micrograms of lymphocytosis promoting factor [LPF] and 12.5 micrograms of filamentous hemagglutinin [FHA]) was compared with conventional pertussis vaccine in a randomized double-blind study involving 40 children aged 4-6 y, 40 children aged 18-24 mo, and 50 infants. Increases in antibody were significantly greater among recipients of acellular vaccine than among recipients of conventional vaccine for antibodies to LPF in all age groups and for antibodies to FHA in infants and children aged 4-6 y; the increase in FHA antibody was also greater with acellular vaccine among children aged 18-24 mo but not significantly so. Compared with conventional vaccine, acellular vaccine was significantly associated with reduced frequency of leg pain and fretfulness at all ages and less frequent fever and anorexia at some ages. The reduced reaction rates and comparable or enhanced immunogenicity of the acellular vaccine make it an attractive candidate for larger field trials, particularly among infants.