THERE has been considerable controversy concerning the origin of new bone in a bone transplant. Some workers1–4 consider that such bone is derived from soft tissues associated with the transplant and is, therefore, of donor origin. Others, however, have suggested that it arises from host tissue metaplasia5. The evidence for these opposing views is extensively reviewed, by Axhausen6. A method of investigating this problem which seems so far not to have been used depends on the immunological relationships between inbred and F1 hybrid animals. It is known from various investigations, particularly of skin7, that whereas tissues of an inbred animal can be successfully grafted to an F1 hybrid animal (which had the inbred concerned as one of its parents), transplantation in the opposite direction induces an immunological response, since the graft possesses antigens (derived from the other inbred parent) which are not possessed by the host.