Progress in recent years in knowledge of respiratory diseases has lead the clinician and laboratory worker to assess at an early stage the disturbances in metabolism of the pulmonary or alveolar tissue. The technique of bronchoalveolar washings (BAW) provides a sample of the alveolar film and opens up the possibility of its biochemical study. The surface tension properties of pulmonary surfactant suggested by Von Neegard then confirmed by Pattle and Clements are due essentially to the presence of phospholipids (lecithins amongst others) and to their association with proteins and other lipids. Analysis of these substances in bronchoalveolar washings and measurement of their fluidifying properties are possible in various forms of pulmonary pathology.