The method for long-term culture of marrow cells in vitro as described by Dexter has recently been successfully applied to human marrow and is dependent on the development of an adherent stromal cell layer consisting of cells described as "endothelial-like cells, fat cells, and macrophages". The present study was designed to determine the origin and composition of the stromal cells forming the in vitro 'microenvironment' and maintaining haematopoiesis in long-term cultures grown from marrows of 14 patients who received marrow transplants from HLA identical siblings of the opposite sex. The presence of a Y chromosome was used as a marker to establish the donor or recipient origin of the cells. We found that the stromal cells became progressively donor in origin with time after transplantation and some reacted with antibody directed against factor VIII-associated antigen. In addition, donor-derived in vitro stromal cells synthesized both interstitial and basal lamina collagen types, indicating that the in vitro microenvironment is transplantable and composed in part of endothelial-like cells.