In cement lines and in adjacent Haversian systems from bone of orangutan and whale the content of mineral, organic matrix, and sulfur was determined microradiographically and the arrangement of the mineral part was studied by X-ray diffraction. Cement lines were often highly mineralized, maximum being 50% by volume. This value was a few per cent higher than the highest value for Haversian systems. Content of collagen varied to a certain extent, and low values were found in cement lines. Determinations of sulfur and X-ray diffraction studies of the organization of the mineral fraction did not reveal any significant differences between cement lines and adjacent osteons.