Earlier analyses of the physical chemistry of lung surfactant (LS) are rejected on the grounds that the definition of surface tension is not complied with. Furthermore, with 1 or 2 exceptions the devices used to measure the properties of LS have given misleading information. On the other hand, a protein-free formulation of an artificial lung-expanding compound (ALEC) consisting of a 7:3 mole/mole mixture of dipalmitoylphosphatidylcholine and phosphatidylglycerol seems to function as a reasonably good substitute for natural LS in very premature babies. It has the following necessary properties. It spreads rapidly and spontaneously at an air/water interface at 37°C, reducing the surface tension of water by about 2/3. The unsaturated phosphatidylglycerol (PG) moiety can be squeezed out of the mixed monolayer by rapid overcompression (equivalent to exhalation) and is irreversibly lost to the surface system by reassembly into liposomes. The residual dipalmitoylphosphatidylcholine (DPPC) becomes progressively enriched to the point that, at 37°C, it condenses out as a solid phase so rigid that it prevents the alveolae from collapsing. The preparation is protein-free.