Syngeneic, allogeneic and xenogeneic (rat) freshly isolated bone marow cells + stromal cell cultures maintained in vitro for 10-30 days, as well as non-adherent cells removed from these cultures on 3rd-4th day were injected into the kidney parenchyma of mice, immunosuppressed with hydrocortisone. In syngeneic grafts the immunosuppression was omitted. In all transplant systems bone tissue was formed inside the kidney with 20% to 32% variation. Bone produced by allogeneic and xenogeneic cells is subject to rejection when immunosuppression ceases, as the bone formed is of donor origin. The "floating" cells, regardless of the transplant system, normally discarded during media replacement, turned out to be efficient bone producers. This notion is of practical implication when bone marrow cells are used for bone healing.