Local transendoscopic endobronchial antigen challenge, which has proved to be a valuable clinical and research technique in the study of human pulmonary hypersensitivity, was evaluated in control and asymptomatic COPD--affected horses. Transendoscopic endobronchial challenges with phosphate-buffered saline (PBS), Micropolyspora faeni extract at 60 and 600 micrograms/ml and mouldy hay extract elicited neutrophilic airway inflammatory responses in control (N = 5-7) and asymptomatic COPD-affected (N = 5-7) horses, as determined by cytological examinations of bronchoalveolar lavage fluid (BALF) harvested from the challenged lung segments. Endobronchial challenges with 600 micrograms M. faeni extract/ml induced a significant BALF neutrophilia only in horses with asymptomatic COPD, when compared with PBS challenges. However, as the BALF neutrophil ratios of COPD-affected horses after this M. faeni challenge did not differ significantly from those of control horses, this finding has little clinical diagnostic value. The BALF neutrophilia induced in control and asymptomatic COPD-affected horses by 60 micrograms M. faeni extract/ml and mouldy hay extract challenges was not significantly different from that induced by PBS challenge. Endoscopically visible bronchial changes were observed in some of the control and COPD-affected horses within 5 mins and at 5 h after PBS, M. faeni and mouldy hay extract challenges. We conclude that this technique is of no value in the investigation of equine COPD.