It has been recognized that middle ear does not represent as immunologically ""previleged"" site and immune responses are a factor in the pathogenesis of otitis media. For the observation of the response to antigenic challenge to the middle ear, rats were immunized systemically by intraperitoneal injection with 1mg ovalbumin and allowed 3 weeks to develop systemic immunity. The subjects were challenged with a single injection of 0.1mg ovalbumin through the tympanic membrane. The middle ear fluid and specimens were harvested at 3rd day, 1st week, 2nd week and 3rd week after challenge. The bulla was entered inferiorly, and 0.1ml saline was instilled and free fluid was aspirated. The maximum amount of middle ear fluid was observed at first week after challenge and fluid almost disappeared at 3rd week after challenge. In the histologic findings, the promontory mucosa showed marked mucosal and submucosal hyperplasia with extensive tissue edema and capillary dilatation at 1st week after challenge and was nearly normalized at third week after challenge. The levels of plasmin, plasminogen activator and plasmin inhibitor, which are main components of fibrinolytic system, in middle ear fluid were assayed and the presence of plasmin, which is a biologic inflammatory mediator, was confirmed. It is concluded that middle ear fluid and inflammation are mediated by antigenic challenge to the middle ear in previously sensitized rat and the presence of plasmin was confirmed. The relationship between plasmin and pathogenesis of otitis media should be necessarily confirmed in further study.