The combinations of a biologic bone replacement material (BRM), bone gelatine (BG), a matrix extract, with one of the synthetic BRMs, either powderised beta-tricalcium-phosphate (TCP) or particulate hydroxyapatite (HA), as well as BG alone were implanted in muscle pouches and diaphyseal femoral drill-holes in rats. Implantation between one and 21 days. Evaluation by histomorphometry of PMMA embedded undecalcified specimen. In heterotopic implantation bone formation occurred earliest in the BG/TCP combination with highest values for volume density bone and osteoid after 21 days. In orthotopic implantation starting between days 9 and 13 the BG/HA combination stimulated intense woven bone formation with massive ingrowth of vital bone into the HA pores. The biodegradable TCP, by activating the monocyte-/macrophage-system for its degradation, obviously enhanced the already ongoing immunologic reaction to allogeneic BG, thus impeding bone formation and repair.