Two acellular pertussis vaccines, a mono-component toxoid of pertussis toxin and a two-component vaccine containing both the toxoid and filamentous haemagglutinin, were used for primary immunization of infants 5 1/2 to 10 months of age in two clinical trials in Sweden. Over a follow-up period of 12 to 17 months, 37 children were exposed to pertussis in the household or a daycare centre. Only one child developed mild, culture-confirmed pertussis as opposed to the expected number of 30 to 33 cases based on an assumed attack rate of 80-90% in non-immunized children. These preliminary findings indicate that, given as primary immunization to infants, the vaccines provide protection against pertussis.