• During a randomized trial of acellular pertussis vaccines, significantly fewer recipients of a two-component vaccine (Japanese National Institute of Health [JNIH]-6) were diagnosed as primary or coprimary cases in households than either placebo recipients or those who received a monocomponent pertussis toxoid vaccine (JNIH-7). After household exposure to a culture-confirmed primary case, efficacy for JNIH-6 was estimated to be 35% (95% confidence interval, −14% to 57%) against any culture-confirmed disease and 58% (95% confidence interval, −6% to 84%) against clinical disease with 21 days or more of coughing spasms. The corresponding efficacy estimates for JNIH-7 were 67% (95% confidence interval, 32% to 80%) and 82% (95% confidence interval, 41% to 96%). Differences between the JNIH-6 and JNIH-7 vaccines in efficacy after household exposure were not statistically significant. No association could be established between protection against pertussis after household exposure and serum levels of IgG antibody to pertussis toxin or filamentous hemagglutinin in vaccinated individuals, in either study children or other household members.