Since the discovery of the extra-cellular lining material of the lung and the possibility harvesting this source by endobronchial lavage this material has been the object of many studies directed to analyze its components, function and possible change in the diseased lung. The best known component of the extra-cellular lining material is the phospholipid and its fatty acid composition. But also on the cellular material much emphasis has been taken with the aim using its cytology as diagnostic parameter. However, very few informations were obtained about the protein material also washed out during the endobronchial lavage. As it was demonstrated by immunological methods the proteins of the extra-cellular lining material consist of serum identical proteins and those being obviously specific for the lung tissue. As found, most serum identical proteins occur in the same amounts as found in the blood serum, and the molecular weight in general range up to 160,000 daltons indicating that there must be a restriction in passage of high molecular weight proteins through the lumen walls of the endothelium. Some proteins, IgG, IgA, do occur in a higher level in the extra-cellular lining material leading to the suggestion that these proteins were synthesized and secreted by the lung tissue itself. The molecular weight of the lung specific proteins range from 16,000-340,000 daltons. Under reducing conditions however, for all species listed, two classes of subunits -36,000 and 12,000 daltons --result, indicating that these proteins might have comparable functions in the different species.(ABSTRACT TRUNCATED AT 250 WORDS)