ABSTRACT After heterotopic (e.g. subcutaneous) transplantation of bone marrow, haemopoiesis in the graft ceases; reticular tissue develops instead, and later bone is formed (Denis, 1958). The result can be achieved by grafting either free pieces of bone marrow or those placed in diffusion chambers (Petrakova, Tolmacheva & Friedenstein, 1963; Rosin, Freiberg & Sajnek, 1963). In the case of free transplantation the bone formed is later filled with bone marrow. After transplantation in diffusion chambers haemopoiesis does not recur despite the development of a considerable mass of bone in the chambers (Friedenstein, 1965).