1. A study of normal and osteoarthritic hyaline cartilage has been made with the electron microscope and x-ray diffraction.

2. Normal cartilage consists of a three-dimensional network of collagen fibrils with no preferred orientation, surrounded by a matrix containing polysaccharide.

3. In the osteoarthritic joint the collagen fibrils show definite orientation and a decreased proportion of ground substance. X-ray diffraction confirms this and shows the orientation to be at right angles to the surface of the femoral head.

4. Tensional forces across the joint may explain why osteoarthritic changes first appear in the non-weight-bearing area of the joint.