A group of anticardiolipin antibodies (aCL) require beta 2-glycoprotein I (beta 2GPI) to recognize their target, which might be located on endothelial cells (EC) and/or platelets. Following incubation with epithelial cells, 13 of 30 lupus sera retained EC-reactive antibodies of the IgG, IgA and IgM isotypes. Associated aCL and anti-phosphatidylethanolamine antibodies were partly absorbed on eC as well as EC. The former antibodies were more efficiently removed in the presence than in the absence of the latter. The presence of beta 2GPI in the affinity-purified aCL preparations may explain their binding to EC, as this cross-reaction was abrogated by the removal of the cofactor and restored by its re-introduction. Seventy four per cent of EC were faintly stained with polyclonal or monoclonal antibody directed to the cofactor. The beta 2GPI mediated aCL binding to EC membranes could this be influential in the development of thrombosis and/or thrombocytopenia in aCL-positive patients.