To clarify the adsorption mechanism of proteins onto calcium hydroxyapatite (Hap), the present study measured adsorption (Delta H-ads) and desorption (Delta H-des) enthalpies of bovine serum albumin (BSA; isoelectric point (iep) 4.7, molecular mass (M-s) 67200 Da, acidic protein), myoglobin (MGB; iep = 7.0, M-s = 17800 Da, neutral protein), and lysozyme (LSZ; iep = 11.1, M-s = 14600 Da, basic protein) onto Hap by a flow microcalorimeter (FMC). Five kinds of large platelike particles of CaHPO4 center dot 2H(2)O (DCPD) after hydrolyzing at room temperature with different concentrations of NaOH aqueous solution ([NaOH]) for 1 h were used. DCPD converted completely to Hap after treatment at [NaOH] >= 2%, and the crystallinity of Hap was increased with an increase in [NaOH] up to 10%. The amounts of protein adsorbed (Delta n(ads)) and desorbed (Delta n(des)) were measured simultaneously by monitoring the protein concentration downstream from the FMC with a UV detector. The Delta n(ads) values were also measured statically by a batch method in each system. The Delta n(ads) values measured by the FMC and static measurements fairly agreed with each other. Results revealed that Delta DATA was decreased with an increase in [NaOH]; in other words, Delta was decreased with the improvement of Hap's crystallinity, suggesting that the BSA adsorption readily proceeded onto Hap. This fact indicated a high affinity of Hap to protein. This affinity was further recognized by Delta because its positive value was increased by increasing [NaOH]. These opposite tendencies in Delta(BSA)(ads) and Delta(BSA)(des) revealed that Hap possessed a high adsorption affinity to BSA (i.e., enthalpy facilitated protein adsorption but hindered its desorption). The fraction of BSA desorption was also decreased with an increase in [NaOH], confirming the high affinity of Hap to protein. Similar results were observed on the LSZ system, though the enthalpy values were smaller than those of BSA. In the case of neutral MGB, Delta H-ads(MGB) also exhibited results similar to those of the BSA and LSZ systems. However, due to its weak adsorption by the van der Waals force, Delta H-abs(MGB) was small and almost zero at [NaOH] >= 2%. Hence, the fraction of MGB desorption was less dependent on [NaOH].