X-ray diffraction patterns were recorded from fully mineralized and demineralized sections of the cortical bone from cow tibia. The results were used to obtain a measure, d, of the mean side-to-side spacing (equatorial diffraction spacing) of the molecules in the type I collagen fibrils of bone. Mineralized bone yielded a d value of 1.23 nm;​ for the demineralized tissue this value increased to 1.52 nm. Thus it appears that mineralization of bone compacts its collagen molecules within the fibrils.