Influenza vaccines together with pertussis toxin B oligomer (PTB) purified from a culture supernatant of Bordetella pertussis were administered intranasally into mice to test for an adjuvant effect of the PTB. An inactivated virus vaccine and an ether-treated HA vaccine prepared from influenza virus A/Yamagata/120/86 (H1N1) and formulated with PTB, stimulated production of serum haemagglutinin inhibition (HI) antibody and pulmonary and endotracheal secretory IgA antibody to high titres. In addition, mice immunized with the influenza vaccines formulated with PTB were protected against exposure with a challenge virus. These results demonstrate that PTB can enhance the immunogenicity of influenza vaccines administered intranasally.