Concentrations of phosphate ion ([Pi]_f) and calcium ion ([Ca²⁺]_f) liberated from the surface of hydroxyapatite (Ca₁₀ (PO₄)₆(OH)₂; HAP) during the adsorption of dodecyl sulfate ion (DS^-) onto HAP were determined at various concentrations of added salt (NaCl or Na₂HPO₄). The effects of added salt on the relationships among [Pi]_f, [Ca²⁺]f, the concentration of DS^-([DS^-]_f), and the adsorbed amount of DS^- (X_DS-) are discussed. The amount of X_DS- increased monotonously with the concentration of NaCl added ([NaCl]_i). However, the equilibrium concentrations of phosphate ion (Pi) and calcium ion (Ca²⁺) changed in a complex manner with [NaCl]_i. It was concluded that ions on the surface of HAP were partly ion-exchanged with those of sodium dodecyl sulfate and NaCl, and that Ca²⁺ and Na⁺ were bound competitively as counter-ions to DS^- micelles. On the other hand, when Na₂HPO₄ was added to the solution, added Pi (i.e., one of the lattice ions for HAP) had a drastic effect on X_DS- and [Ca²⁺]_f. The adsorbed amount of DS^- decreased with increasing concentration of added Na₂HPO₄ owing to competitive adsorption between DS^- and added Pi. The equilibrium concentration of Ca²⁺, [Ca²⁺]_f, decreased for two reasons. First, adsorbed Pi inhibits the release of Ca²⁺ from the surface of HAP by virtue of the electrostatic attractive force. Secondly, the excess Pi remaining in the solution prevents Ca²⁺-release from the surface of HAP since the solubility product for HAP must remain constant.