The possibility of the formation of a new bone marrow organ from the initially isolated bone marrow cells has been proved for the first time. Suspensions of the bone marrow cells (5 x 10(6)) placed on millipore filters were grafted under the renal capsule in mice. As a result, the bone formed and the medullary cavity developed which was populated by hemopo etc cells. These processes were closely followed during 9 weeks. To compare the sizes of hemopoietic territories formed by grafting of different bone marrow fragments, the number of bone marrow cells in the heterotopic bone marrow was estimated. The bone marrow organs formed under the renal capsule after grafting bone marrow from the whole femur or its half contained practically the same number of hemopoietic cells. It was also true for the subcutaneous transplants of bone cylinders of the same size but with different amount of bone marrow. Hence it follows that the amount of hemopoietic territory grafted depends to a great extent on the transplant form and, therefore, does not reflect the number of initially grafted microenvironment-forming cells.