Serum was taken from 195 infant pertussis patients several times at appropriate intervals, and antibodies to pertussis toxin and filamentous hemagglutinin were evaluated by the ELISA. The geometric mean titers were low for both IgG antibodies at the onset of the disease, and rose to about 30 U/ml for both antibodies in the convalescent sera. All convalescent sera showed high titers of anti-PT IgG antibody. Paired serum samples were collected from 685 children before and four weeks after the second primary dose of immunization with the component vaccine. The titers were less than 1.0 and 1.8 U/ml for anti-PT and anti-FHA IgG, respectively for pre-immunization, and 22 and 66 U/ml for anti-PT and anti-FHA IgG, respectively for after immunization. When children six months of age were immunized with the component vaccine, good antibody responses were seen and no significant differences of antibody response by age were observed. There were 71 children who were immunized twice with the component vaccine and were considered to be exposed to infection by household members. Nine children out of 71 had weak or typical attacks of pertussis, and the attack rate was about 13% (9/71). It was concluded that the newly developed vaccine is suitable for children with respect to immunogenicity, and it was assumed that 20 to 30 ELISA U/ml of anti-PT and anti-FHA IgG is sufficient for protection from pertussis.