Whether bone marrow stromal cells of donors contribute physiologically to hematopoietic stem cell reconstitution after marrow transplantation is unknown. To determine the transplantability of nonhematopoietic marrow stromal cells, stable clonal stromal cell line (GB1/6) expressing the a isoenzyme of glucose-6-phosphate isomerase (Glu6PI-a, D-glucose-6-phosphate ketol-isomerase; EC 5.3.1.9) was derived from murine long-term bone marrow cultures and made resistant to neomycin analogue G418 by retroviral gene transfer. GB1/6 cells were fibronectin+, laminin+, and collagen-type IV+ and collagen type I-; these GB1/6 cells supported in vitro growth of hematopoietic stem cells forming colony-forming units of spleen cells (CFU-S) and of granulocytes, erythrocytes, and macrophage/megakarocytes (CFU-GEMM) in the absence of detectable growth factors interleukin 3 (multi-colony-stimulating factor), granulocyte/macrophage colony-stimulating factor, granulocyte-stimulating factor, or their poly(A)+ mRNAs. The GB1/6 cells produced macrophage colony-stimulating factor constitutively. Recipient C57BL/6J (glucose-6-phosphate isomerase b) mice that received 3-Gy total-body irradiation and 13 Gy to the right hind limb were injected i.v. with GB1/6 cells. Engrafted mice demonstrated donor-originating Glu6PI-a+ stromal cells in marrow sinuses in situ 2 mo after transplantation and a significantly enhanced hematopoietic recovery compared with control irradiated nontransplanted mice. Continuous (over numerous passages) marrow cultures derived from transplanted mice demonstrated G418-resistant, Glu6PI-a+ stromal colony-forming cells and greater cumulative production of multipotential stem cells of recipient origin compared with cultures established from irradiated, nontransplanted control mice. These data are evidence for physiological function in vivo of a transplanted bone marrow stromal cell line.