A study was undertaken of the acute effect of the inhalation of ozone (1.0-1.5 ppm) on the concentration of proteins in the airway secretions harvested from the trachea of intact anesthetized dogs. Secretions were first collected during a control day during which room air was breathed. The dogs breathed ozone (1.0-1.5 ppm) on a second day. This protocol permitted each dog to serve as its own control. During the inhalation of ozone the concentration of total aqueous-soluble proteins, as well as of albumin and IgG were not significantly effected. However, when expressed as a percentage of total protein, both albumin and IgG were significantly enriched in the secretions. The quantitative changes of certain proteins in airway secretions may be an important reflection of the pulmonary response to ozone.