The hematopoietic system of vertebrates can be completely reconstituted with hematopoietic stem cells derived from the bone marrow, fetal liver, or cord blood, or even from peripheral-blood-derived cells. A cellular marker to identify those cells is the proteoglycan CD34, although we have shown that the earliest identifiable hematopoietic stem cell is a CD34(-) fibroblast-like cell which can differentiate into CD34(+) hematopoietic precursors. Peripheral blood mononuclear cells were isolated from the heparinized blood of a dog and incubated in tissue culture in the presence of interleukin 6. After 10-14 days, an adherent layer of fibroblast-like cells had developed and cells were immortalized using the SV-40 large T antigen. Cells were cloned and subcloned by measures of limiting dilution, and various fibroblast-like clones were established. These fibroblast-like cells either do not express the CD34 antigen or express CD34 on a low level, although transcribing CD34. The CD34(-/low) cells express osteocalcin as a mesenchymal cell marker. The fibroblast-like cells eventually differentiate spontaneously in vitro into CD34(+) precursors and show colony formation. Prior to autologous stem cell transplantation, one clone of choice (IIIG7) was transfected with a retroviral construct containing the green-fluorescence protein (GFP). The recipient dog was totally irradiated with 300 cGy and received a stem cell transplant with GFP-containing, immortalized, fibroblast-like monoclonal autologous stem cells (0.5 x 10(8)/kg dog). No additional growth factors were applied. The peripheral blood counts recovered after 23 days (WBC >500; platelets >10,000). A peripheral blood smear showed some dim but definite, although timely, limited expression of the GFP protein in nucleated peripheral blood cells just five weeks after transplantation. A bone marrow biopsy showed GFP-positive cells in the marrow cavity predominantly as "bone-lining cells."