High resolution Ca-43 solid-state NMR studies of hydroxyapatite (Ca-10(PO4)(6)(OH2)) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using H-1-Ca-43 rotational echo double resonance and H-1-Ca-43 cross polarization magic angle spinning experiments.