Vasculitis is an common clinical feature of systemic lupus erythematosus (SLE) in humans and in animal models of this disease. Humoral autoimmunity against endothelial cells has been previously demonstrated in SLE and other autoimmune disorders, but the precise cell surface antigenic targets involved in the initiation and progression of vascular injury are still essentially unknown. In the current studies, we demonstrate the presence of autoantibodies in the sera of MRL/lpr/lpr mice which bind endothelial cell surface antigens by ELISA and also cause complement-dependent cytotoxicity of these cells. These MRL/lpr/lpr sera induced complement-dependent cleavage and release of 35SO4-labeled material containing primarily cell surface heparan sulfate proteoglycans from these cells, and react with heparin (a glycosaminoglycan related to heparan sulfate) by ELISA and liquid-phase competitive inhibition ELISA. These data indicate that antiendothelial cell autoantibodies present in autoimmune MRL/lpr/lpr mice are directed at least in part against cell surface heparan sulfate proteoglycans. Autoantibodies to cell surface heparan sulfate proteoglycan may play a role in vascular endothelial cell injury in these animals through complement-dependent, autoimmune mechanisms.