Small bone sticks from a rat and a dog, fixed in formaldehyde from one to several weeks, were used as homografts and heterografts in defects created in the mandibles of rats. The animals were killed 30, 60, 90, and 150 days after the graft implantation procedure. Histologic study showed that although the homografts induced a distinct osteogenic activity and were incorporated by the host in less than 90 days, the heterografts behaved as foreign bodies and became encapsulated by connective tissue and remained unresorbed up to 150 days. Therefore, the clinical applicability of formaldehyde-fixed homografts should be considered.