In a randomized double-blind trial 55 children of 15-24 months and 56 children of 4-6 years of age previously immunized with whole-cell DTP (WC-DTP) received acellular pertussis DTP vaccines containing 12.5 micrograms (AC-12.5) or 25 micrograms (AC-25) each of pertussis toxoid (PT) and filamentous haemagglutinin (FHA) per dose of WC-DTP. No differences in antibody responses or adverse events were noted for children who received AC-25 as compared with AC-12.5. All three groups had significant increases in pertussis agglutinins, but the geometric mean titre (GMT) for 4-6-year-old children who received WC-DTP was higher than the GMT for children who received acellular vaccine. No significant differences were noted in the GMT of antibodies to FHA or PT between children who received WC-DTP and recipients of acellular vaccine. The rates of several adverse reactions were significantly (p less than or equal to 0.05) higher for recipients of WC-DTP, and children given WC-DTP were significantly (p less than or equal to 0.00001) more likely to have received acetaminophen. These acellular vaccines are safe and as immunogenic for FHA and PT as WC-DTP when administered as the fourth or fifth dose to children who received three doses of WC-DTP in infancy. The lower (12.5 micrograms) dose of acellular vaccine was as effective as the higher (25 micrograms) dose in inducing antibodies to FHA and PT in children 15-24 months and 4-6 years of age.