Mesenchymal stem cells are multipotent cells resident in the bone marrow throughout adulthood which have the capacity to differentiate into cartilage, bone, fat, muscle, and tendon. A number of monoclonal antibodies raised against human MSCs have been shown to react with surface antigens on these cells in vitro. A protein of molecular mass 92 kDa was immunoprecipitated using the SH-2 monoclonal antibody. This was purified and identified by peptide sequencing analysis and mass spectrometry as endoglin (CD105), the TGF-beta receptor III present on endothelial cells, syncytiotrophoblasts, macrophages, and connective tissue stromal cells. Endoglin on MSCs potentially plays a role in TGF-beta signalling in the control of chondrogenic differentiation of MSCs and also in mediating interactions between MSCs and haematopoietic cells in the bone marrow microenvironment. (C) 1999 Academic Press.