Western blot and agglutination techniques were used to analyze the antibody responses to Bordetella pertussis in 27 infants less than six month of age with presumed pertussis infection. The antibody response to the Bordetella pertussis adhesions filamentous hemagglutinin, pertactin and agglutinogens, and to the Bordetella pertussis toxins pertussis toxin and adenylate cyclase-hemolysin were compared. Infection induced intense antibody responses to filamentous hemagglutinin, pertussis toxin and adenylate cyclase-hemolysin. Antibodies to agglutinogens were never detected, and antibodies to pertactin were rarely detected in infected infants' sera. Therefore, determination of anti-agglutinogens levels only is not suitable for the serological diagnosis of pertussis in young infants. Use of purified filamentous hemagglutinin, pertussis toxin and adenylate cyclase-hemolysin in Western blot analysis may improve the serodiagnosis of Bordetella infections. However, care must be exercised in distinguishing between the antibody response in young infants and maternally derived antibodies.