Although dipalmitoyl lecithin, is the essential component of the pulmonary surfactant system that is invoked for alveolar stability, there is no explanation as yet for the origin and role of certain proteins that are found with the phospholipid in pulmonary washings. Aqueous lavages obtained from rabbit lung contain three major proteins, two of which are serum proteins (albumin 60%, gamma-globulin 10%), and the third, protein "T" (20%), is described here as pulmonary secretory immunoglobulin A (sIgA1. The protein is first recovered quantitatively in the surface active lipid-protein fraction from filtration of pulmonary lavage on Sephadex G-200. The protein is then isolated from the lipid either by filtration on SDS-Sephadex G-200 or by ethanol-ether precipitation. After reductive cleavage with either mercaptoethanol or dithiothreitol and alkylation with iodoacetamide, three protein peaks are obtained from SDS-sephadex G-200 separation. The products of reductive cleavage are analogous to the secretory component (MW approximately 60,000), H-chain (MW approximately 50,000), L-chain (MW approximately 25,000), and J-piece (MW approximately 28,000) of secretory IgA from colostrum. In immunodiffusion the lung protein and colostrum sIgA show striking identity lines, as do antibodies to rabbit colostrum and to the lung protein. Amino acid and carbohydrate analyses reveal some differences between the two secretory immunoglobulins. Although this protein has been found together with the phospholipid surfactant of the lung in vitro, the present structural study concludes that the protein is sIgA. Although concurrent immunofluorescence studies showed that this protein is in the alveolar lining layer, we cannot as yet conclude that it belongs to the surfactant system of the lung.