This paper investigates the parameters of transplants determining the size of heterotopic bone marrow organs (HBMO's). The mice were irradiated in doses of 1.5-7.0 Gy on a cobalt source with dose rate of 28.2 rads/min ten minutes before removal of the marrow. After 2.5 months each HMBO was removed, a suspension was prepared from the cells of its medullary cavity, and the number of cells was counted in a Goryaev's chamber. The results were subjected to statistical analysis. Under all conditions the results showed that no hypothetical linear relationship exists between the size of HBMO and the number of transplanted medullary cells. It has been suggested that during heterotopic bone marrow transplantation a fraction of surplus or reserve carriers of the microenvironment arises.