Interferon (IFN)-γ knockout (IFN-γ^-/-) mice and background BALB/c (IFN-γ^+/+) mice were intranasally immunized with outer membrane proteins P6 purified from nontypeable Haemophilus influenzae, and Th1/Th2 and B cell immune responses were compared to investigate the role of Th1-type cells in inducing mucosal IgA responses against H. influenzae in the nose. P6 dissolved in PBS was administered on days 0, 7, and 14. Antigen-specific antibody titers in serum, saliva, nasal wash, and fecal extract were determined by ELISA. On day 21, mononuclear cells isolated from mucosal tissues were analyzed by P6-specific ELISPOT. CD4⁺ T cells isolated from spleen were cultured with P6 for 4 days, and the concentrations of cytokines in the culture supernatants were examined by ELISA. Further, the number of cytokine-producing cells were determined by cytokine-specific ELISPOT. P6-specific IgA antibody titers in saliva, nasal wash, and fecal extract and the numbers of P6-specific IgA-producing cells in nasal passages and submandibular gland were lower in IFN-γ^-/- mice than IFN-γ^+/+ mice. Although IFN-γ^-/- mice did not produce IFN-γ, the production of IL-2 and IL-6 was observed in the same manner as in IFN-γ^+/+ mice. These findings suggest that IFN-γ plays an important role in inducing P6-specific IgA responses in nasal mucosa.