Mesenchymal stem cells (MSCs) are defined as cells that can differentiate into multiple mesenchymal lineage cells. MSCs have some features (surface molecules and cytokine production, etc.) common to so-called traditional bone marrow (BM) stromal cells, which have the capacity to support hemopoiesis. In the present study, we isolated murine MSCs (mMSCs) from the fetal BM using an anti-PA6 monoclonal antibody (mAb) that is specific for bone marrow stromal cells. The mMSCs, called FMS/PA6-P cells, are adherent, fibroblastic, and extensively expanded and have the ability to differentiate not only into osteoblasts and adipocytes but also into vascular endothelial cells. The FMS/PA6-P cells produce a broad spectrum of cytokines and growth factors closely related to hemopoiesis and show good hemopoiesis-supporting capacity both in vivo and in vitro, suggesting that they are a component of the hemopoietic stem cell niche in vivo. Interestingly, although the FMS/PA6-P cells express a high level of the PA6 molecule, which is reactive with anti-PA6 mAb, they gradually lose their ability to express this molecule during the course of differentiation into osteoblasts and adipocytes, indicating that the PA6 molecule might serve as a novel marker of mMSCs.