The source of stromal cells following bone marrow transplantation has been investigated by culturing marrow cells obtained from patients who have a graft cells from a donor of the opposite sex. Two culture systems have been used for these studies. The first supports the proliferation of fibroblast colony-forming cells (F-CFC) and the formation of confluent layers of fibroblastoid cells; the second (the long-term bone marrow culture system) results in the formation of a complex and heterogeneous layer of adherent cells composed of fibroblasts, fat cells, endothelial cells, macrophages and 'blanket' cells. A survey of the work done using these culture systems shows that, although the issue of the transplantability of stromal cells remains controversial, it now seems reasonably certain that repopulation of the fibroblast component by donor cells is rare. In contrast, one study has shown that endothelial-like cells can engraft in transplant recipients. Another study has demonstrated a donor origin for the macrophages found in long-term cultures of marrow from transplanted patients. This finding might have been predicted because macrophages are derived from haemopoietic stem cells.