Dacron vascular prostheses were examined under both light and electron microscopes from 1 to 1184 days after implantation in the thoracic aorta of 96 dogs. Each Dacron fiber of the implanted prostheses was found to be covered with a single layer, which closely resembled the outer half leaflet of plasma membrane. The control experiment in vitro revealed that when untreated Dacron fibers were dipped into a solution of lecithin, the fibers became covered with monomolecular leaflets of lecithin micelles. On the basis of the in vivo and in vitro findings, it was suggested that the implanted Dacron fibers of vascular prosthesis were covered at an early stage with a monomolecular micelle layer of phospholipid or phospholipoproteins originating from the living body, and thus became adapted to the living body.