Human mesenchymal stem cells (MSC) were seeded onto the inner surface of a tubular silicon construct and, after 24 h, were exposed to a shearing stress of either 2.5 or 10 dyne/cm(2) for 1 day. The fluid contained endothelial growth factors in both cases. Morphological changes and cytoskeletal rearrangements were observed in the stimulated cells. Immunofluorescence staining showed that low (2.5 dyne/cm(2)) and high shear stress (10 dyne/cm(2)) resulted in the expression of von Willebrand factor (vWF) and calponin, respectively. At low shear stress, CD31 (PECAM-1) was significantly expressed whereas vWF and KDR expression was only slightly higher than those under 10 dyne/cm(2). All three markers related to smooth muscle cells (myocardin, myosin heavy chain, and SMaEuro22 alpha) had significantly higher expression under shear stress of 10 dyne/cm(2) compared with a 2.5 dyne/cm(2), even in endothelial growth medium. Shear stress plays a critical role in regulating MSC differentiation and must be considered for bioengineered blood vessels.