ARTICULAR cartilage consists principally of collagen fibres embedded in a mucopolysaccharide ground substance. Electron microscopy studies1 show that the collagen fibres near the articular surface are predominantly parallel to the surface; viewed perpendicularly to the surface, they also show a dominant orientation which varies systematically over the whole joint surface. In contrast, the fibres in the deeper zones have a more random distribution with a tendency to be perpendicular to the surface. The fibre diameter and distance between adjacent fibres appear to increase with depth from the articular surface. Chemical and physico-chemical studies (unpublished results of H. Muir and A. Maroudas) confirm that the collagen density decreases with depth from the articular surface.