We were interested in whether ozone (O3) could stimulate the migration of mast cells into the tracheal lumen. To test this we determined the effect of an acute O3exposure on the types and relative numbers of cells recovered by tracheal lavage. Seven conscious sheep were intubated with an elongated nasotracheal tube. The trachea between the larynx and the cuff of the tracheal tube (15–20 cm) was lavaged repeatedly with 10–15‐ml aliquots (total 350 ml) of 0.9% buffered (pH 7.4 saline, which contained the mast cell‐stabilizer disodium cromoglycate (10 μg/ml). One hour after a baseline lavage, the sheep were exposed on separate occasions to either air (control) or 0.5 ppm O3for 2 h. Lavages were repeated 24 h later. Cells were recovered from the lavage effluent by centrifugation across a saline/Ficoll Paque gradient. From part of this material we estimated total cells and total viable cells (with Trypan blue). The rest of the material was recentrifuged at 400 × g for 5 min, and cytological slides were made from the cell pellet. Slides were stained with Poli‐chrome and Wright‐Giemsa, and were analyzed by light microscopy. The percentages of epithelial cells, macrophages, lymphocytes, and mast cells were determined from a total count of 500 cells/slide. Differences in cell percentages between pre‐and postexposure were calculated both for air and O3exposures, and these differences were compared. Exposure to O3resulted in an increased number of mast cells and lymphocytes when compared to the changes observed with air. It seems likely that the increase in number of luminal mast cells and lymphocytes following O3exposure signals an enhanced inflammatory response and that these changes could contribute to O3‐induced increased nonspecific airway hyperresponsiveness and susceptibility to allergic IgE‐mediated airway reactions.