We have studied the osteogenic effects of nacre (mother of pearl) placed in experimental cavities prepared in the lumbar vertebrae of sheep. Some of cavities were filled with nacre, some with PMMA, and some were left empty. The vertebrae were removed 1, 8, 12 weeks after surgery, and assessed histologically and morphometrically. The nacre particles in the bone cavity and the surrounding intertrabecular spaces gradually dissolved beginning at 8 weeks after surgery. There were layers of newly formed bone, both woven and lamellar, in various stages of maturation in contact with or adjacent to the dissolving nacre. Quantitative assessment of the activation of bone formation adjacent to the cavities filled with nacre indicated significant activation of bone formation, which continued until week 12. There was also increased mineralization of the host bone at this time. There was no new bone formation in the empty cavities, or in those filled with PMMA. PMMA also caused necrosis of surrounding bone cells with a change in bone architecture and significant reductions in bone formation and mineralization. This study demonstrates that nacre stimulates bone-forming cells in vertebrae and appears to result in new bone formation.