Autoimmune CL-rA need a plasma protein, beta 2GPI, to bind anionic PL. While beta 2GPI could be the true antigen, beta 2GPI binding to solid phase heparin did not determine its recognition by CL-rA when using patient plasmas. We tested plasmas from four patients with antiphospholipid syndrome in this study and no CL-rA binding to heparin sepharose was obtained. On the contrary, when CL-rA were first purified from the plasmas by means of a CL-octyl sepharose column, the purified material bound to the heparin sepharose column. Thus, after recognizing beta 2 GPI, CL-rA bind heparin. 'In vivo' CL-rA binding to heparin like-substances could inhibit the anti-thrombotic properties of endothelial cells.