The air-blood barrier was studied in replicas of freeze-fractured lung biopsies collected from healthy human subjects. Adjacent pneumocytes display a belt-like network composed of 3-7 superimposed ridges ("fibrils") on the P face and complementary grooves on the E face, i.e., a structure corresponding to a "tight" junction. On the other hand, adjacent capillary endothelial cells show a continuous system of 2-4 membrane foldings. These appear mainly as smooth surfaced crests on the P face; on the E face furrows are seen, at the bottom of which a row of particles is situated. This arrangement suggests a "leaky" type of junction. Discontinuous occluding junctions are located in the pericytic venular segment of the alveolar vessels. The present findings are in agreement with previous physiological and ultrastructural tracer studies locating the main part of the diffusion barrier for small polar solutes and proteins in the alveolar epithelium. Communicating junctions are demonstrated between type I and type II pneumocytes, indicating intercellular cooperation between these cells of common embryonic origin, but which fulfill different functions in the adult. In the endothelium of the non-muscular alveolar vessels communicating junctions are lacking. Desmosomes occur in the epithelium between type I and type II pneumocytes; square arrays of particles characterize the plasma membrane of type I pneumocytes.