Coaxial bioassay system (guinea-pig trachea and rat anococcygeus muscle as donor and bioassay organs, respectively) and tracheal open ring preparations from ovalbumin-sensitized guinea-pigs were used to investigate the role of the epithelium in response to the antigen challenge. Ovalbumin induced concentration-dependent relaxations in the phenylephrine precontracted rat anococcygeus muscle suspended in the lumen of sensitized guinea-pig tracheal tube preparations in the presence of either indomethacin or mepacrine. Removal of the epithelium abolished ovalbumin-induced relaxation responses. In tracheal open ring preparations, epithelium removal shifted the ovalbumin concentration-response curve to the left in both the control and mepacrine-treated experiments. Indomethacin significantly potentiated the ovalbumin-induced contractions in epithelium-intact preparations, while in epithelium-denuded ones no significant change was observed. In the presence of indomethacin, the maximum contractile responses were significantly higher in epithelium-intact preparations than in epithelium-denuded ones. These results provide direct evidence for the release of epithelium-derived relaxant factor(s), which is (are) neither a cyclooxygenase nor a lipoxygenase product, in response to the antigen challenge. In addition, the observation of the increased maximum contraction to antigen in epithelium-intact preparations in the presence of indomethacin, suggests the involvement of contractile influences derived from the epithelium in antigen-induced responses of airways.