Composites composed of microcrystalline calcium-deficient hydroxyapatite (HAp) and collagen were formed at 38Β°C via an acid-base reaction between calcium phosphate precursors in the presence of a collagen matrix. Formation of composites having HAp:collagen weight ratios of 4.5:1, 11:1, and 221, along with that of pure mineral were investigated. Isothermal calorimetry and X-ray diffraction indicated complete reaction within 5 h resulting in hardened monoliths. The rate of HAP formation increased with an increase in the proportion of collagen present. Electron microscopy indicated that the acceleratory effect of collagen was associated with the provision of nucleation sites for HAp crystallization. Analysis of the solution chemistry also showed that collagen affected the calcium and phosphate concentrations and the pH. While collagen was shown to effect the kinetics of HAp formation, the rate limiting step, as shown by X-ray diffraction and solution chemistry, was the dissolution of the acidic calcium phosphate precursor, CaHPO,. Preliminary mechanical data indicated that the Young's modulus, yield strength, and work to fracture were at the lower end of the range of those values reported for bone. The porosities observed in these composites suggest that they might be osteoinductive while their compositions should allow their eventual resorption. Thus, microstructure, kinetics, and mechanical data suggest that these composites might be suitable as bone substitutes which form in vivo. 0 1995