Changes in tracheal epithelial surfaces induced by Mycoplasma infection in vivo and in vitro included release of mucous granules followed by exfoliation of ciliated and nonciliated epithelial cells. Light microscopy, scanning electron microscopy, and transmission electron microscopy confirmed that the loss of cilia from individual cells was infrequent. Epithelial cells typically lost their intercellular connections, rounded up, exfoliated, and then lysed--giving rise to a population of cellular organelles, such as mitochondria and cilia intermixed with mucus to form the exudate found within the tracheal lumen. Repair of the epithelial surface was effected by basilar epithelial cells differentiating and filling in the spaces formed by exfoliated cells. These cells were hypertrophied, nonciliated at 14 days after infection in vivo, and covered with microvilli. In sectioned material obtained during the infection, there was increasing epithelial thickness due to cellular infiltration and edema. Tracheal rings in vitro showed similar changes to those seen in vivo, except that exfoliation was more severe and occurred earlier. In addition, there were no cellular infiltration due to the lack of a vascular supply and only a small amount of mucus due to the smaller number of mucous cells available to release into the tracheal lumen.