3801 children aged 5-11 months were entered into a blind placebo-controlled trial of pertussis vaccine. 954 were randomised to receive placebo (vaccine solvent), 1419 to receive a two-component vaccine containing formaldehyde detoxified lymphocytosis promoting factor (LPF) and filamentous haemagglutinin, and 1428 to receive an LPF-toxoid vaccine. After 7-13 weeks 3724 infants received a second dose. Immediate side-effects were mild. Small local reactions occurred more often in the vaccinated infants than in those who received placebo, especially after the second dose of the two-component vaccine. During 15 months of follow-up from 30 days after the second dose, culture-confirmed whooping cough (cough and a positive culture of Bordetella pertussis) occurred in 40 placebo, 27 LPF-toxoid vaccine, and 18 two-component vaccine recipients. The point estimate of protective efficacy was 54% (95% confidence intervals 26-72%) for the LPF-toxoid vaccine and 69% (47-82) for the two-component vaccine; protection against culture-confirmed whooping cough of over 30 days duration was 80% (59-91%) and 79% (57-90%), respectively.