To assess the safety, immunogenicity, and lot consistency of a five-component acellular pertussis vaccine combined with diphtheria and tetanus toxoid (Connaught Laboratories Limited), we randomly allocated 432 infants to receive one of three lots of an acellular pertussis vaccine or a single lot of whole cell pertussis vaccine. Infants were immunized at 2, 4 and 6 months of age and between 17 and 19 months of age. Local and systemic adverse reactions were reported significantly more frequently by recipients of the whole cell than acellular vaccine after each dose. The antibody response against pertussis toxin, filamentous hemagglutinin, and 69 kDa protein was of greater magnitude in acellular pertussis vaccine recipients than whole cell pertussis vaccine recipients. Small differences were detected amongst the vaccine lots tested. We conclude that the acellular pertussis vaccine is safe and immunogenic for the first four doses in children under 2 years of age.