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To study the specificity of serum antibodies against filamentous hemagglutinin (FHA) and pertactin for infection with Bordetella pertussis, we followed the acquisition of IgG serum antibodies against these 2 surface proteins of the organism in children who had been vaccinated with a monocomponent pertussis toxoid vaccine and who had experienced no symptoms of pertussis. Antibodies were estimated with enzymelinked immunosorbent assay. In Part 1 of our study 5 consecutive samples obtained between 3 and 36 months of age from 71 children were available. Most had maternally derived antibodies to FHA (70 of 71) and pertactin (51 of 71) in the 3-month sera which declined in the subsequent sera. From about 1 year of age there were small but significant increases in antibodies against both antigens. At 3 years of age 71 of 71 had antibodies to FHA and 58 of 71 had antibodies to pertactin. In Part 2 of our study sera from 109 three-year old children were available. The 12 children with a history of family exposure to pertussis had significantly higher geometric mean titers of FHA antibodies than the 97 children with no history of family exposure. The geometric mean titers of pertactin antibodies did not differ. We suggest 3 explanations for the acquisition of FHA and pertactin antibodies in children with no history of pertussis: (1) asymptomatic B. pertussis infection in vaccinated children; (2) infection with Bordetella parapertussis; (3) infection with cross-reacting antigens from other organisms, e.g., nonencapsulated Haemophilus influenzae.