Hydroxylapatite. a synthetic bone mineral, was implanted alone or as a composite with either calcium sulfate (plaster of Paris) or Avitene (microfibrillar collagen hemostatic agent) into the orbital bony detects and orbital soft tissues of 23 rabbits. Plain Avitene was also placed into these bony and soft tissue sites and hone defects with no additives served as controls. Additionally, each implant material was placed into subcutaneous tissues to determine its relative inflammatory potential. New bone formation was noted in bony delects as early as 10 days. Bone ingrowth was greatest when hydroxyl-apatite was in direct apposition to bone. Material implanted away from bone showed predominately connective tissue ingrowth. Minimal inflammatory reaction was noted early on in the hydroxylapatite and hydroxylapatite/calcium sulfate samples and had resolved by 2 months when placed in both soft and bony tissues. Samples of Avitene and the hydroxylapatite/Avitene composite demonstrated an intense inflammatory response, and in several cases large granulomas were seen after 2 months.