We examine the six-month histology of a quick-setting hydroxyapatite cement (Mimix), with and without absorbable plates (LactoSorb), after placement in critical size defects made in the skull of 20 New Zealand white rabbits. Two 10 mm diameter parietal defects were made in each rabbit. One defect was placed on each side of the sagittal suture. In 10 animals, the left defect was filled with Mimix alone, and in the other 10 an absorbable plate was placed across the base of the left defect prior to Mimix placement. The right-sided defect served as a control in all animals, with no material placed in or across this defect. The rabbits were sacrificed six months postoperatively. A bone pathologist performed histologic examinations of the treated specimens looking specifically for evidence of inflammatory reaction and bone formation. There was no inflammation around the dura or any other negative histologic reaction in either of the study groups or the control. Growth of new bone into the Mimix was found along the periphery in all our specimens. We found no migration or displacement of the Mimix from any of the calvarial defects.