In vitro-cultured bone marrow cells have been shown to contain some low-density lipoprotein (LDL) uptake-positive cells. Although a small portion of LDL uptake-positive cells had expression for endothelial markers, all of them demonstrated a phagocytosis function similar to monocyte/macrophages and expression of the panleukocyte surface marker CD45 and monocyte marker CD14. These LDL uptake-positive cells did not show significant proliferative capacity and died out gradually in long-term culture. In contrast, the bone marrow-derived LDL uptake-negative cells showed strong proliferation and expression of typical mesenchymal surface markers CD29 and CD44. Although cultured under endothelial promoting conditions, these mesenchymal stem cells (MSCs) did not show any sign of differentiation toward endothelial cells. In conclusion, adult bone marrow-derived LDL uptake-positive cells that have been reported so far actually are monocytes/macrophages that can express some endothelial markers but are not "true endothelial progenitor cells" (EPCs). MSCs, which are the only cell type that shows strong proliferation during long-term adherent culture for bone marrow cells, do not differentiate toward the endothelial lineage when grown under endothelial promoting conditions.