Previous measurements of the hydroxyl (OH^–) ion content of the calcium phosphate crystals of bone mineral have indicated a substantial depletion or near-absence of OH^–, despite its presumed status as a constituent of the hydroxyapatite lattice. Analytical methods for determining bone crystal OH^– content have depended on procedures or assumptions that may have biased the results, such as chemical pretreatment to eliminate interference from the organic matrix. We demonstrate a two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy technique that detects the proton spectrum of bone crystals while suppressing the interfering matrix signals, eliminating the need for specimen pretreatment other than cryogenic grinding. Results on fresh-frozen and ground whole bone of several mammalian species show that the bone crystal OH^– is readily detectable; a rough estimate yields an OH^– content of human cortical bone of about 20% of the amount expected in stoichiometric hydroxyapatite. This finding sheds light on the biochemical processes underlying normal and abnormal bone mineral metabolism.