Despite the relatively high incidence of defects found in the graft 6 months after the operation, we feel that this heterograft (Kiel bone) is a useful addition in the vertebral fusion operations. Macro and microscopic examinations carried out 6 months following the operation showed good incorporation of the Kiel bone in those cases in which rectangular pieces of cancellous Kiel bone were placed in a spread-out fashion on the top of and between the slivers of autogenous bone. Our limited experience bears out the fact that it is unwise to place the strip sof Kiel bone in contact with each other and neither should they be placed on the non osseous tissue. In those latter instances the incorporation of the Kiel bone is slow and apparently not complete, at least not within the period of 6 months following the operation. The Kiel bone does not provide a substitution for the fresh autogenous transplant in spinal fusion operations, but it provides a very useful addition to the autogenous graft.