The effects of anesthesia on pulmonary antibacterial defense mechanisms of the murine lung were assessed quantitatively. Following infection with aerosols of32P-labelled S. aurcus, mice were anesthetized to approximately equivalent levels with halothane, pentobarbital, methoxyflurane or cyclopropane. After four hours of anesthesia, the animals were sacrificed and the rate of radiophosphorus removal and intrinsic pulmonary bactericidal activity were determined by established techniques. Anesthesia with methoxyflurane or cyclopropane significantly depressed pulmonary bactericidal activity. Anesthesia with halothane or pentobarbital did not affect bactericidal activity. None of the anesthetics altered the rate of physical removal of bacteria. The adverse effects of methoxyflurane and cyclopropane could not be attributed to changes in body temperature or blood-gas composition.