The amount of concurrent adsorption (i. e. co-adsorption) of Ca²⁺ and OH^- on hydroxyapatite (HAP) was determined. It was found that the amount of adsorption of OH^- (or Ca²⁺) increases linearly with that of the other co-adsorbate, Ca²⁺ (or OH^-). Analysis of the data showed that there are two different adsorption sites on the surface of HAP : one is for individual ionic adsorption and the other is for ion-exchange and neutralization of proton. The adsorption isotherms for OH^- (or Ca²⁺) were reduced to one curve when they were displaced somewhat in parallel with a straight line of slope -1, taking the adsorbed amount of co-adsorbate, Ca²⁺ (or OH^-), into consideration. Adsorption of OH^- and Ca²⁺ and/or coordination of Ca²⁺ to the surface phosphate on HAP are considered to be similar to what occurs during the crystal growth of HAP.