Natural abundance 43 Ca solid-state NMR of hydroxyapatite (Ca-10(PO4)(6)(OH)(2)) was performed at three different fields (8.45,14.1 and 18.8 T). The two crystallographically distinct calcium sites of the apatite structure were spectroscopically resolved at 18.8 T. The 43 Ca NMR interaction parameters (delta(iso), C-Q and eta Q) of each site were determined by multiple magnetic-field simulations. The peaks with delta(iso) = 11.2 +/- 0.8 and -1.8 +/- 0.8 ppm, both with C-Q = 2.6 +/- 0.4 MHz, were assigned to the Ca(II) and Ca(I) sites, respectively, on the basis of their relative intensities. Copyright (C) 2008 John Wiley & Sons, Ltd.