An in vitro analysis was conducted to investigate the hypothetical antibacterial and antimycotic activity of the common anesthetic gases (halothane, enflurane, isoflurane, methoxyflurane) in view of the clinical absence of bronchopulmonary pathology after inhalation narcosis despite the many risk factors involved. For this purpose scalar dilutions of the four gases were prepared on cultures of Klebsiella pneumoniae and Candida albicans and the antibacterial action of the gases was tested in vitro. Even with the weaker concentrations used, halothane and methoxyflurane totally inhibited both microorganisms. Enflurane had less effect on Klebsiella p. and almost none on Candida. Isoflurane, a new halogen ether anesthetic was found to have an excellent inhibitory effect. In conclusion it is hypothesised that the anesthetic gases considered might have an in vivo antibacterial activity considering the experimental results obtained in vitro.