To explore the role of protein kinase C (PKC) in the augmented bronchial smooth muscle (BSM) contraction observed in the antigen-induced airway hyperresponsive (AHR) mice, the effects of a PKC activator, phorbol 12,13-dibutylate (PDBu), on BSM contraction were compared between the AHR and control mice. Actively sensitized mice were repeatedly challenged by antigen inhalation. Twenty-four hours after the final antigen challenge the isometrical contractions of the BSMs were measured. The BSM contraction induced by acetylcholine, but not high K⁺ depolarization, was significantly augmented in the AHR mice. In BSMs of control mice, PDBu caused a significant increase in tension when the tissues were precontracted with high K⁺, although PDBu itself had no effect on basal tone. The PDBu-mediated contraction was markedly augmented in BSMs of the AHR mice. These findings suggest that an increase in the PKC-mediated signaling is involved in the augmented contraction of BSMs in the antigen-induced AHR mice.