The biological activity of different volume mixes of boneSafe with MCB in a critical sized rabbit model was discussed. Bilateral distal femoral defects were created in 20 female New Zealand White rabbits and filled with different bone graft materials. It was found that the empty defect was indeed critical sized. New bone formation was only found at the margins of the defect. It was also observed that the mixes of of BS and MCB are usable as bone graft extenders, but the final evaluation should be done in a clinical realistic and loaded model.