Autoimmune antiphospholipid antibodies are associated with thrombosis, thrombocytopenia, and recurrent fetal death. These antibodies are thought to be directed against an epitope formed by phospholipids complexed with beta 2 glycoprotein I (beta 2GPI), a plasma protein with affinity for phospholipids. By immunizing mice and rabbits with heterologous beta 2GPI, we produced antibodies that bind strongly to acidic phospholipids in addition to binding to beta 2GPI. To evaluate the specificities of these antibodies, we passed the serum of a rabbit immunized with beta 2GPI through a cardiolipin (CL) affinity column and studied the bound and fall-through antibodies for binding to CL and beta 2GPI by ELISA. The results demonstrated the presence of two populations of antibodies, one with specificity for beta 2GPI alone without binding to phospholipids and the other with specificities for both CL and beta 2GPI. These dual-specificity antibodies are similar to the antiphospholipid antibodies present in autoimmune diseases because they bind to other acidic phospholipids in addition to CL and because their binding to phospholipids is enhanced by beta 2GPI and blocked by placental anticoagulant protein I (annexin V).