Human alveolar basement membrane (ABM) has been isolated and compared with human glomerular basement membrane (GBM) and trophoblast basement membrane (TBM). The three basement membranes (BMs) differed substantially in chemical composition. ABM had an intermediate hydroxyproline content but contained little hydroxylysine; elevated proline, alanine, and valine and the presence of desmosines indicated that elastin remained associated with this membrane. ABM contained less sialic acid and hexosamine than the other two membranes. Of the three BMs, ABM was the most difficult to solubilize; dilute alkali solubilized 12% of ABM; reduction and alkylation, 43%; plasmin digestion, 24%; collagenase digestion 42%, and elastase digestion, 78–83%. Soluble material from all these procedures gave reactions of identity among the three BMs in immunodiffusion gels with antisera raised against all three BMs. Since the anti-human BM antisera required absorption with human plasma, canine lung and kidney were perfused until blood-free; ABM and GBM were prepared from the perfused organs. Antisera raised against the canine ABM did not react with canine plasma; antigens unique to the lung (not present in kidney) were detected with this system. The canine ABM was also solubilized somewhat by alkali (13%) and plasmin (29%) and more completely by elastase (88%). Since plasmin digested BMs it may play a role in connective tissue remodeling.