Porous blocks of carbonate apatite (CA) were prepared by holding together CA particles ranging in size from 300 to 500 microm through sintering at 750 degrees C for 2 hours. Bone marrow cells taken from Fischer rats were seeded onto and inside the CA blocks and cultured for 14 days to allow stem cells to proliferate to osteoblasts capable of inducing bone formation. Hybrids made of CA blocks and cultured bone marrow cells were then implanted into the back of syngeneic rats. Microfocus x-ray computed tomographic images of tissues containing CA blocks before decalcification suggested that new bone was formed in this extraosseous site 4 and 8 weeks after implantation. These data indicate that the hybrid made of CA and bone marrow cells is capable of inducing heterotopic bone formation in vivo.