The distribution of IgG antibodies to Bordetella pertussis was investigated in serum samples from 550 subjects, aged 4-24 years, to determine the optimal age for booster immunisation. Levels of antibody to B. pertussis antigens were determined using an ELISA that measures a mixture of pertussis toxin, filamentous haemagglutinin and lipopolysaccharide. Geometric mean titres of anti-pertussis antibodies in subjects aged 4-6 years were significantly lower than those in other age groups, which reflects waning immunity following vaccination. High positive titres in older children and adolescents suggested acquired B. pertussis infection, and booster doses at the ages of 7 and 15 years are therefore suggested.