By using tritiated thymidine as a cellular label, it has been shown that when the femora of inbred newborn mice are transplanted as isografts, both the cells of the graft and of the host contribute to the new-bone formation that occurs in the ensuing two weeks. In the case of the homografts, the new-bone formation in this time interval is limited and is derived primarily from the cells of the graft rather than from the host.