Histological investigations were performed on 24 polyethylene terephthalate ligaments (eight Dacron, eleven Trevira and five Ligapro) which had served as substitute for the human anterior cruciate ligament. Because of rupture or loosening they had been explanted six to 48 months after operation. Only in one Trevira ligament a longitudinal direction of collagenous lamellae could be seen, all other ligaments had only few collagenous fibers, mainly orientated in circular direction. In the fibrous tissue of all three kinds of ligaments there were signs of inflammation. Foreign body giant cells have immigrated--they mainly enclose the graft fibres in Dacron and Ligapro ligaments. In Trevira and Ligapro ligaments the foreign body giant cells phagocyte particles of graft fibers after increasing implantation times. The inflammatory reaction to graft fibers and their particles may also damage host tissue. Regarding these results we conclude that the Polyethylene terephthalate ligaments are not qualified as substitutes for the anterior cruciate ligament.