This study was performed to determine whether the process of wound healing, following periodontal surgery, could be improved through the combined use of collagen and grafting of hydroxyapatite (HAP) particles. Twenty-four proximal defects were made in the mandibular fourth premolars and second molars of six adult mongrel dogs. Steel wires and resin were put into the defect to enhance plaque formation. At eight weeks, the wires and resin were removed. At ten weeks, HAP or HAP-collagen complex was implanted during reconstructive surgery, along the root surface treated with acid conditioning. Dogs that received no implant following a flap operation served as controls. In these three groups of animals, differences in the extent and features of healing were histopathologically examined two months later. Animals implanted with a HAP-collagen complex showed a larger amount of new cementum formation when compared with HAP-implanted or control animals. In addition, in animals from the HAP-complex group, the interdigitation between the root surface and the gingival connective tissue fibers tended to be reinforced resulting in suppressed epithelial downgrowth. However, neither bone formation nor the reformation of the periodontium was promoted in the HAP-collagen complex group. These results suggest that implantation of an HAP-collagen complex promotes cemento-genesis of the demineralized root surface and can establish a stronger interdigitation between the root surface and the gingival connective tissue fibers.