In what is thought to be the first application of the technique in the hard tissue field, neutron diffraction study of human tooth enamel has reconfirmed that the inorganic portion is basically an hydroxyapatite-like material. However, compared to hydroxyapatite, the tooth enamel shows ( i) an apparent 20 per cent or greater deficiency of hydroxyl hydrogen, with probable substitution of other ions on or near the hexad axis, and ( ii) an overall excess of hydrogen. Amorphous calcium phosphate, octacalcium phosphate, adsorbed water and structural hydrogen in unusual locations are shown not to be present in major degree, though the possible existence of minor components of OCP (~2 wt%) and hydrogen involved with PO 4 groups (e.g. 1 wt%) are not ruled out. It is therefore left to a combination of residual protein content and entrapped free water to account for the substantial quantity of hydrogen present external to the hydroxyapatite component. The particular usefulness of neutron diffraction and scattering techniques for study of the amount, location, and, possibly, binding of the hydrogen present, whether or not structurally incorporated, is pointed out.