A cell-free alveolar extracellular matrix has been prepared from the lungs of several species of young and adult animals by a procedure consisting of extraction of parenchymal fragments with the detergents Triton X-100 and sodium deoxycholate. The isolated extracellular matrix preparations were ultrastructurally heterogeneous consisting of amorphous basement membranes and associated interstitial collagen and microfibrillar components as shown by histochemical staining and their electron microscopic appearance. The amino acid and carbohydrate compositions of these preparations had a collagenous nature which resembled in many respects that of some ultrastructurally pure basement membranes. Urea extraction of extracellular matrix from adult animals solubilized a distinctly less collagenous fraction which was particularly rich in the acidic amino acids, aspartic and glutamic acids, and had a chemical similarity to acidic structural glycoproteins. Extraction of adult samples with ureamercaptoethanol and pepsin digestion of the insolubilized residues gave more collagenous fractions. Our ultrastructural and chemical examination of these alveolar extracellular matrix preparations clearly indicate that they cannot properly be designated as pure alveolar basement membranes. These findings and the anatomical characteristics of the alveolar blood-air interface make it probable that preparations isolated by others using sonication procedures, with chemical characteristics similar to those reported here, were not ultrastructurally pure alveolar basement membranes as reported but were actually heterogeneous mixtures of basement membranes, interstitial collagens and microfibrils. It would appear that the alveolar basement membrane cannot be isolated in a form comparable to ultrastructurally pure basement membranes such as that of the renal glomerulus. © 1981 Springer-Verlag.