The reactivity of alpha-tricalcium phosphate (alpha-TCP) in forming hydroxyapatite (HAp) at 37degreesC was investigated. The effects of synthesis route, HAp seeding and the presence of calcium salts on the mechanism and extent of HAp formation were examined by pH measurements and/or isothermal calorimetric analyses. A synthesis temperature at the lower end in the temperature range of 1100-1300degreesC and the reaction of alpha-TCP with a high specific surface area greatly improved rate and extent of HAp formation. The time for complete reaction decreased from 18 h to 14 h, when the reaction was carried out in the presence of 1 wt% of HAp seeds; the hydrolysis mechanism did not change. At HAp seeds proportion of 5 wt% and 10 wt%, transformation occurred without a nucleation period. The calcium salt additives studied were anhydrous and dihydrate form of dicalcium phosphate (CaHPO4 and CaHPO4 . 2H(2)O), calcium carbonate (CaCO3) and calcium sulfate hemihydrate (CaSO4 . 1/2H(2)O). All the additives delayed HAp formation as determined by the isothermal calorimetric analyses. Their retarding effects in decreasing order are CaCO3, CaSO4 . 1/2H(2)O, DCPD, DCP. CaCO3 almost completely retarded HAp formation. After 24 h, hydrolysis was complete only for pure alpha-TCP and for the alpha-TCP-DCP blend. Reaction was complete in other formulations before 48 h except for the CaCO3-Containing blend. In all mixtures conversion to HAp occurred without forming any intermediates. However gypsum formed in the mixture containing CaSO4 . 1/2H(2)O. All the alpha-TCP-additive mixtures, excluding alpha-TCP-CaCO3, reached nominally the same strength value after 24 h of reaction as governed by the transformation of alpha-TCP to HAp. For phase-pure alpha-TCP, the average tensile strength changed from 0.36 +/- 0.03 MPa to 7.26 +/- 0.6 MPa. Upon hydrolysis only the CaSO4 . 1/2H(2)O-containing mixture exhibited slightly higher strength averaging 8.36 +/- 0.9 MPa.