AbstractThe study of collagen fibers by X‐ray diffraction, utilizing semiquantitative indices for appraisal of “structuration” and “orientation” was applied to 30 aortic valve grafts. These grafts, of pig origin, were studied in the fresh state, after tanning with aldehydes, and after having been implanted in patients for at least 18 months. It was shown that the collagen fibers are preferentially orientated parallel to the transverse axis of the valve cusps, and that this arrangement, enhanced by tanning, had a tendency to disappear in the post‐implantation samples. The structuration index which concerns the fibril network was also augmented by tanning, and was found a little high or slightly diminished in post‐implantation cusps. Evidence for a new compound, probably fibrin, was found in the diffraction patterns of implanted cusps, but crystallized calcium was noti