Tracheal cilia of cows exposed, in vitro, for 20 hours to different acidities and alkalinities of sulfuric acid and sodium hydroxide, respectively, showed ciliomotility at pH values as low as 4.9, while the epithelial cells began to be expelled from the mucosa at pH 6.7, initiating a reduction of ciliary function around this pH value. In alkaline reactions, ciliostasis occurred at pH 9.76. Destroyed cilia were found above pH 10.15, while they were morphologically unchanged at pH values as low as 4.0, the lowest value examined. The first symptoms, however, of adverse effects were intracellular edema and the simultaneous occurrence of cellular polyps in both acid and alkaline reactions at pH 6.7 and 9.5, respectively. The results indicate that the early effects of air pollutants are better demonstrated by transmission electron microscopy studies than by, e.g., studies of ciliomotility.