The filamentous protein component of amyloid-laden tissue was studied by x-ray diffraction procedures. The principal features of the x-ray pattern from nonoriented amyloid material consist of a sharp, intense ring at 4.75 Å overlaying a diffuse halo at 4.3 Å, and a broad and less intense ring at 9.8 Å. When oriented, the material gives a "cross-β" x-ray pattern. The x-ray findings are interpreted in terms of a "pleated sheet" structure formed by the amyloid polypeptide chain folding in a regular manner on itself such that adjacent chain segments are laterally arranged in an antiparallel manner. The x-ray patterns from oriented amyloid suggest further that the axes of the chain segments run transverse to the filament axis.