Human endothelial cells were separated by collagenase digestion from the umbilical vein of newborns. The cytotoxic effect of antisera against HLA-DRw antigens was tested on monolayers of endothelial cells, using a51Cr retention microcytotox-icity assay in the presence of rabbit complement. Of the endothelial monolayers tested, significant concordance between the typing results obtained using cord blood-derived B lymphocytes and endothelial cell monolayers was observed. A rabbit anti-human polyspecific B cell antiserum was also capable of completely lysing the endothelial monolayers tested. In addition,5ICr-labelled dissociated endothelial cells were lysed by nonimmune peripheral blood mononuclear cells in the presence of DRw antisera having a specificity for the target endothelium.The vascular network of allografted organs is an important site for immunological injury and endothelial damage is a common histopathological observation in both acute and chronic rejection. Although the mechanism of allograft vascular destruction is presently incompletely understood, damage to endothelial cells in vitro by antibodies via complement-dependent or cell-mediated cytotoxicity has been demonstrated(1–3).Although all nucleated cells carry the HLA-A, B, and C antigens, the tissue distribution of the HLA-DRw antigens appears to be limited primarily to B lymphocytes, monocytes, epidermal Langerhans cells, and possibly sperm (see Ref.(4)). We present evidence here that human endothelial cells also carry the DRw antigens similar or identical to those expressed on human B cells and monocytes, and that anti-DRw antisera in the presence of complement are capable of specifically destroying monolayers of human endothelial cells. In addition, we also describe damage to dissociated human endothelial cells in vitro by nonimmune allogeneic peripheral blood mononuclear cells in the presence of anti-HLA-A, B, or DRw antibodies. A preliminary report describing part of our studies will appear elsewhere (5).