When untreated porous calcium phosphate ceramics were transplanted into subcutaneous (s.c.) or intramuscular (i.m.) sites, fibrovascular tissue grew in the pore region without evidence of bone formation. However, when these same ceramics were combined with syngeneic marrow cells, osteogenesis was observed inside the pore region of the implanted ceramic. The osteogenesis began on the surface of the pore region at approximately 3 weeks postimplantation by a process of intramembranous bone formation, with the de novo bone tissue observed directly interfacing with the ceramic surface. Infrequently, small isolated areas showed cartilage formation with no noticeable endochondral ossification. At 4 weeks postimplantation of the ceramic with marrow cells, the osteogenesis in the ceramic accompanied an observed increase in compressive strength, rigidity, and energy absorption of the ceramic. These results suggest that a combination of porous ceramics and marrow cells may be useful for clinical problems requiring osseous reconstruction.