Well-crystallized hydroxyapatite was synthesized at 80 degrees C and pH 7.4, and mixed as filler with 2.2'(4-methacryloxydiethoxyphenyl) propane. BPO and DHPT were used as polymerization initiators. The compressive strength and Knoop hardness of the apatite composite resins increased with the increase of apatite content, and approached a plateau above an apatite-resin ratio (Ap/R) of 1. The thermal expansion coefficient of the composite with apatite-resin ratio Ap/R = 1 was almost equal to that of teeth. In tooth cavities, the composites seemed to adhere well to enamel without a bonding agent. The biocompatibility of the composites implanted hypodermically into rats appeared to improve with the increase of apatite content, and implants showed no significant inflammation after 1 wk and 4 wk.