Four children with pertussis and their 18 family members were subjects of a 1-year study to detect infection and antibody responses to Bordetella pertussis. Attack rate for pertussis infection in contacts was 83%. Two-thirds of cases in these immunized contacts were subclinical. All infected family contacts had diagnostically elevated serologic tests for pertussis at the time the index case was diagnosed. Culture identified only 20% ofinfected contacts. Infected individuals had a mean of 5.5 of 10 antibody tests diagnostic for recent infection. ELISA assay for IgG to pertussis toxin and assay for IgA to filamentous hemagglutinin on serum and nasal secretions were the most discriminating diagnostic tests. Index cases and immunized contacts had different type and timing of antibody responses, making a single assay or sampling unable to identify all infected individuals. Symptomatic infection was characterized by higher magnitude of pertussis toxin antibody response and asymptomatic infection by filamentous hemagglutinin. After pertussis immunization, immunity to disease is greater than is protection from infection.