The alveolar wall on contact with air can be compared with a biological air-liquide interface. As with all interfaces, there are therefore superficial forces which tend to reduce the surface to a minimum. In the case of a pulmonary alveolus with a spherical surface, these forces are at the origin of an internal pressure excess dependent on the radius of the alveolus and on the superficial tension related to the nature of the interface. Owing to the disparity in the alveolar radii, under these conditions the smaller alveoli would collapse to the benefit of a larger one, the pressure being lower and lower in the latter. In addition, at any time in the respiratory cycles, this surpression must be negligible in order to avoid rupture of the equilibrium of the forces exerted on the alveolar wall. Consequently, it is necessary that this air-alveolar wall interface should have a superficial tension on the one hand variable with the surface, and on the other hand always very low. Owing to the demonstration of large concentrations of phospholipid in this area, it can be thought that a superficial film is substituted at the air-biological liquid interface and owing to this fact effectively has superficial properties necessary for alveolar stability. The "surface-tensio-active" effect of this film lead to the giving of the name of "surfactant" to these constituents as a whole.