Numerous chemical analyses of the acellular lung lining layer have been made and there is little doubt concerning the identity of the surfactant material obligatory for normal lung function. However, the acellular layer is also a barrier to the environment and consequently may be one of the first sites subject to attack by inhaled toxic agents. Unfortunately, apart from its lipid composition very little is known about the acellular lining layer. The objectives of this investigation are (1) to determine the enzymatic and protein composition of the lung lining, (2) to determine the origin of these activities, and (3) to investigate compositional variations of the lining under the influence of drugs, disease and airborne pollutants.