The serum antibody responses of vaccinated children and whooping cough patients to the highly purified Bordetella pertussis antigens, leukocytosis-promoting factor (LPF), lipopolysaccharide (LPS), a protein binding to complement-fixing antibodies induced during whooping cough (PBCA) and to a partly purified filamentous hemagglutinin fraction (FHA) were analysed in enzyme-linked immunosorbent assay. Of the IgG antibodies, those to FHA and LPS were often persistent both after infection and vaccination. The mean titers were about six to ten times higher after disease than after vaccination in corresponding age groups. IgG antibodies to LPF were frequently detected in high titers in patients (mean arbitrary units = 1723), but seldom in low titers (mean units = 20), after vaccination. IgG antibodies to PBCA disappeared some years after vaccination. IgM antibodies to PBCA, FHA and LPS were present in almost 100% after vaccination and disease. IgM antibodies to LPF were detected in 23% of the vaccinated and in 83% of the patients. The respective mean IgM units to PBCA, FHA, LPS and LPF were 14, 13, 16 and 134 times higher in patients than in vaccinated children. None of the vaccinated children and 20% of the patients had IgA antibodies to LPF. No pronounced differences in the percentage of the respective IgA responses to PBCA, the FHA fraction and LPS were found between the two groups. The striking differences in the immune response of vaccinated children and patients were to LPF. Since whooping cough protects better than vaccination against B. pertussis infection, the present study indicates that immunogenic LPF should be included in an acellular vaccine. Also addition of FHA, PBCA and possibly even of LPS, if it can be prepared in an immunogenic and atoxic form, might be necessary in order to prepare a highly effective acellular vaccine.