Isolated human mesenchymal stem cells which can differentiate into more than one tissue type (e.g. bone, cartilage, muscle or marrow stroma), a method for isolating, purifying, and culturally expanding human mesenchymal stem cells (i.e. "mesenchymal stem cells" or "hMSCs"), and characterization of and uses, particularly research reagent, diagnostic and therapeutic uses for such cells. The stem cells can be culture expanded without differentiating. Monoclonal antibodies specific for human mesenchymal stem cells and the monoclonal hybridoma cell lines (ATCC nos. 10743-10745) that synthesize and secrete these monospecific antibodies, and uses of the monoclonal antibodies for diagnostic and/or therapeutic purposes.