Endothelialization of the inner face of a prosthesis appears to improve patency of small caliber arterial substitutes. The importance of understanding the factors that affect human endothelial cell behavior is highlighted by failure of vascular prosthetic grafts to endothelialize when implanted in man. In the present study, endothelial cells isolated from microvasculature are used for their ability to be easily selected from human adult fat, their proliferative capacity, and for their immunologic properties relevant to human pathology: allograft implantation, vessel injury or atherosclerosis. The system described provides a tool for assessing the individual roles of shear stress in modulating endothelial cell morphology and major histocompatibility complex (MHC) antigen expression. Using indirect immunofluorescent staining, initial results showed a homogenous increase of class I and appearance of class II expression after an exposure for 30 hr to physiologic arterial values. Significantly increased staining intensity was observed following exposure to supraphysiologic values. Moreover, precoating of substrate with fibronectin instead of poly-L-Lysine enhanced MHC straining intensity. Scanning electron microscopy (SEM) confirmed the activated morphology of stained cells. This provides a model to study involvement of MHC expression in endothelial cell activation under physical constraints. It may contribute to the development of biomaterial for implantation.