The safety and immunogenicity of two formulations of an acellular pertussis vaccine as a booster at 17 to 19 months of age were assessed in children immunized at 2, 4 and 6 months of age with acellular or whole cell pertussis vaccine. In Study I 86 children primed with a five-component acellular vaccine combined with diphtheria and tetanus toxoids or with a whole cell pertussis-diphtheria-tetanus vaccine were boosted with the same vaccine. Local reactions (64% vs. 93%; relative risk, 0.7; 95% confidence interval, 0.5 to 0.9) and systemic reactions (68% vs. 97%; relative risk, 0.7; 95% confidence interval, 0.5 to 0.9) were less common after the fourth dose of acellular vaccine than after the fourth dose of whole cell vaccine. In Study II 96 children primed with either an acellular or whole cell pertussis vaccine were boosted with an acellular vaccine. Local adverse reactions after booster immunization with acellular vaccine were more common in children primed with acellular vaccine than those primed with whole cell vaccine (68% vs. 33%; relative risk, 2.1; 95% confidence interval, 1.3 to 3.3). Antibody response to pertussis toxin, filamentous hemagglutinin and fimbriae were higher before and 1 month after the booster dose in children primed with the acellular vaccine. We conclude that the acellular pertussis vaccine is safe and immunogenic when used for the booster dose in children primed with either whole cell or acellular vaccine but is associated with local reactions.