Currently, there is considerable interest in the possibility of using cultured human bone marrow stromal cells (BMSCs) for skeletal tissue engineering. However, the factors that regulate their ex vivo expansion and promote their osteogenic maturation remain poorly defined. Using BMSCs obtained from a large cohort of adult donors, the effects of transforming growth factor (TGF)beta1 on these processes have been determined. BMSCs were found to express TGFbeta receptors (TbetaRs) I, II, III (betaglycan) and CD105/endoglin. The expression of TbetaRs I and II, but not TbetaR III or endoglin, was linked to the cells' state of maturation. Treatment with TGFbeta increased the colony-forming efficiency (CFE) of marrow cell suspensions but reduced the median diameter of the colonies that formed and the number of cells harvested at the end of primary culture. Treatment with TGFbeta also resulted in a significant downregulation in the expression of the developmental markers alkaline phosphatase (AP) and STRO-1. The reduction in AP was due to a decrease in the absolute number of cells expressing this enzyme and in the level (sites/cell) at which it was expressed. Overall, the changes in the expression of STRO-1 and AP are consistent with TGFbeta acting to decrease the size of the osteoprogenitor fraction, and hence the potential clinical utility of the cultured cell population.