The activation of endothelial cells is thought to contribute to the host response to infection and to the pathogenesis of autoimmune disease. It was recently shown that antibody and complement can activate endothelial cells leading to cleavage and release of heparan sulfate from the cells. We show here that release of heparan sulfate from endothelial cells is mediated by antibody and the complement fragment C5a and that assembly of the membrane attack complex and lysis of endothelial cells is not necessarily involved. These data suggest that the generation of C5a in conditions such as autoimmunity and infection in which anti-endothelial cell antibodies may also be present, might amplify tissue injury by a novel mechanism involving endothelial cell activation and loss of heparan sulfate mediated by antibody and C5a.