Protein and fat-free bone sludge were 2-stage calcined in rotary kiln at 600-950 degrees C to produce hydroxyapatite analyzed for chem. compn., structure and sp. surface. The Ca, total P, As, Cd, Hg, Co, Cr and Ni contents in the product increased with increasing calcination temp. while the contents of Pb, Cu and acid-soluble P decreased. The heavy metal contents remained below the allowable concns. of the elements. The sp. surface and pore size decreased also with increasing calcination temp.