Since bone marrow has been shown to contain cells capable of differentiating into various lineages, it is attracting considerable attention as a potential source for tissue regeneration. The plasticity of hematopoietic stem cells (HSC), however, remains controversial. While recent studies have suggested that HSC give rise to other tissue types, others argue against HSC plasticity. Because any tissue injury results in extensive recruitment of inflammatory cells, evidence for stem cell plasticity with non-specific green fluorescent protein (GFP) transgenic mice is often confounded by the results of adhesion and/or cell fusion by macrophages. Some reports using non-specific GFP transgenic mice demonstrated that HSC could contribute to the generation of smooth muscle type α-actin (αSMA)-expressing cells in injured arteries or renal gromeruli. We developed a new transgenic αSMA-GFP mouse with GFP coupled to an αSMA regulatory sequence. Bone marrow transplantation was then used to evaluate the potential of marked cells to generate donor type tissues in irradiation chimeras. There was a highly restricted pattern of GFP expression in the transgenics, marking vascular smooth muscle cells, pericytes and mesangial cells in the kidney, but no hematopoietic cells while the marrow contained infrequent cells positive for αSMA-GFP (0.02% of mononuclear cells). When the marrow cells were cultured, stromal cells with high GFP expression were abundantly produced in two weeks. Similarly cultured glomeruli generated many mesangial cells with high GFP expression. These characteristics, however, were not transferable to lethally irradiated (950rad) RAG-2 deficient mice, even when 1 x !07 transgenic marrow cells per recipient were infused via tail vein and the hematopoietic system was largely replaced by recipient cells. At 6 months after transplantation, no GFP+ cells were observed in either bone marrow or renal glomeruli of 20 RAG-2 deficient recipients. Furthermore, cultivation of their marrow or glomerular cells did not produce αSMA-GFP expressing cells. Our findings support earlier studies suggesting that the bone marrow microenvironment is difficult to transplant by standard methods and indicate that HSC are unlikely to give rise to αSMA expressing progeny.