Anticardiolipin antibodies (aCL) in the sera of patients with antiphospholipid syndrome (APS) recognize an altered structure of beta 2-glycoprotein I (beta 2-GPI) interacting with solid-phase negatively charged phospholipids. beta 2-GPI bound to Cu2+-oxidized plasma lipoproteins, i.e. oxidized very low-density lipoprotein (oxVLDL), oxidized low-density lipoprotein (oxLDL), or oxidized high-density lipoprotein (oxHDL). beta 2-GPI inhibited in vitro uptake, i.e. cell surface binding, cellular association, and proteolytic degradation of oxLDL by murine macrophage J774A.1 cells. The binding of oxLDL to the macrophages was inhibited by the addition of polyinosinic acid (poly (I)), a competitor of the scavenger receptor, but not by another polyanionic acid, polycytidylic acid (poly (C)). Conversely, the binding of oxLDL was significantly increased by the simultaneous addition of human beta 2-GPI and monoclonal aCL derived from NZW x BXSB F1 (WB F1) mice, an animal model of APS, or anti-beta 2-GPI antibodies from BALB/c mice immunized with human beta 2-GPI. These findings indicate that beta 2-GPI may be an antiatherogenic protein and that the autoimmune response against beta 2-GPI may have a role in the development of atherosclerosis in APS.