Adsorption to the air-water interface of natural lung surfactant obtained by bovine lung lavage is compared and contrasted with the adsorption of mixtures of synthetic phospholipids and of extracted mixed lung lipids containing minimal protein. Surface pressure-time (pi-t) adsorption isotherms are measured at 35 degrees C for the surfactant mixtures as a function of the presence or absence of divalent metal cations (Ca2+ and Mg2+) and of heating to 45 degrees C or 90 degrees C. The effect of aqueous dispersion technique (sonication or mechanical vortexing) on the adsorption process is also studied for the extracted or synthetic phospholipid mixtures. The results imply that the protein component is necessary for the optimal adsorption of natural lung surfactant. However, by taking advantage of different methods available for phospholipid dispersion in an aqueous phase in vitro, it is possible to formulate dispersions of extracted lung phospholipids containing of order 1% protein which adsorb as well as the complete surfactant system. These results suggest that protein concentrations in surfactant mixtures can be minimized for applications such as exogenous lung surfactant replacement for the neonatal Respiratory Distress Syndrome (RDS). However, for situations which may involve alterations in endogenous surfactant function such as in lung injury, effects involving pulmonary surfactant protein and protein-lipid interactions may be of functional significance.