Although there has been recent emphasis on autoantibodies to epitopes on beta-2-glycoprotein I and prothrombin in the pathogenesis of antiphospholipid syndrome (APS), antibodies other than those directed toward epitopes on phospholipid binding proteins are present. These include those reactive with antigens on platelet membrane glycoproteins, and with vascular endothelial cell membrane. As the pathogenesis of the thrombotic manifestations of APS remains unexplained, further characterization of these antibodies may be informative. We have confirmed anti-endothelial cell binding to a range of cell membrane antigens in systemic lupus erythematosus (SLE) and primary APS. Furthermore, differences in both the pattern of antibody binding and band intensity between human umbilical vein (HUVEC) and human microvascular endothelial cells (HMEC-1) were demonstrated. Of 17 primary APS sera, antibody binding to HUVEC cell membranes was found in nine and to HMEC-1 membranes in seven. Binding at 72-79 kD was confined to HUVEC. In 32 SLE sera, binding to HUVEC and HMEC-1 membranes was detected in 17 and 22 respectively, binding at 135-155 kD being confined to HMEC-1. These results are consistent with the phenotypic variation in endothelial cells of different origins and confirm the frequent presence of autoantibodies reactive with vascular endothelium in both SLE and PAPS. Whether these antibodies could be involved in the pathogenesis of thrombosis, through induction of endothelial cell apoptosis or damage, remains to be determined.