Endothelial cells lining the vasculature participate in a variety of physiological processes. Following cell activation, functional changes are accompanied by changes in the surface structure (or phenotype) of these cells. Studies to date have tended to concentrate on selective changes induced with one or two surface molecules. The following study uses a different approach, having assessed potential changes to the endothelial cell surface using a large number (> 120) of previously untested monoclonal antibodies, and the cytokines TNF-alpha and gamma-IFN, as well as the proteolytic enzyme thrombin. Antibody representatives from all cluster of differentiation groups CD1 through to CD54 were assessed in these studies, which used human umbilical vein endothelial cells. In line with previous observations, antibodies within CD9, CD13, CD26, CD29, CD31, CD34, CD44, CD46, CD47, CD49, CD51 and CD54 gave significant and consistent reactivity using non-stimulated ('quiescent') endothelium. Using parallel cells differentially stimulated with TNF-alpha, gamma-IFN or thrombin, antibodies within CD1 through to CD15, CDw17 to CD19, CD21 to CD23, CD26, CD27, CD29, CD30, CD33 to CD35, CD37, CD38, CD40, CD43 to CD46, CD48, CD51 to CD53 failed to provide any consistent alteration to reactivity patterns compared to non-stimulated cells. There did, however, appear to be some activation induced changes using antibodies within the other CD groups (i.e. CD16, CD20, CD24, CD25, CD28, CD31, CD32, CD36, CD39, CD41, CD42, CD47, CD49, CD50 and CD54) which ranged from minor to significant in scope and magnitude.