BACKGROUND: It has been anticipated that the amount and composition of mucin are changed in patients with chronic airway diseases. We evaluated whether RTO3 (mAb against rat tracheal mucins) could que ntify the amount of mucin from the airway in the patients with chronic airway diseases. METHODS AND RESULTS: 1) RTO3 was bound to high molecular weight of mucin based on Western blot in sputum and BALF from patients with chronic airway diseases. 2) The goblet cells and submucosal glands in main bronchus from human were observed by PAS stain. And immunohistochemical stain with RTO3 showed immunoreactivity on some goblet cells. 3) The amount of mucin was more increased in patients with chronic airway diseases compared to those in normal subjects. 4) In the exacerbation of asthmatics, mucin amounts were more increased than stable asthmatics. CONCLUSION: We suggested that secreted mucin in chronic airway diseases can be quantified by ELISA with RTO3.