Mesenchymal stem cells (MSCs) derived from bone marrow are an important tool in tissue engineering and cell-based therapies because of their multipotent capacity. Majority of studies on MSCs have investigated the roles of growth factors, cytokines, and hormones. Antioxidants such as ascorbic acid can be used to expand MSCs while preserving their differentiation ability. Moreover, ascorbic acid can also stimulate MSC proliferation without reciprocal loss of phenotype and differentiation potency. In this study, we evaluated the effects of ascorbic acid on the proliferation, differentiation, extracellular matrix (ECM) secretion of MSCs. The MSCs were cultured in media containing various concentrations (0-500 microM) of L-ascorbate-2-phosphate (Asc-2-P) for 2 weeks, following which they were differentiated into adipocytes and osteoblasts. Ascorbic acid stimulated ECM secretion (collagen and glycosaminoglycan) and cell proliferation. Moreover, the phenotypes of the experimental groups as well as the differentiation potential of MSCs remained unchanged. The apparent absence of decreased cell density or morphologic change is consistent with the toxicity observed with 5-250 microM concentrations of Asc-2-P. The results demonstrate that MSC proliferation or differentiation depends on ascorbic acid concentration.