Anticardiolipin (aCL) antibodies were isolated from systemic lupus erythematosus serum using cardiolipin liposomes. In a sensitized activated partial thromboplastin time (aPTT) system, low concentrations of aCL were found to significantly prolong aPTT in a dose-dependent fashion. This effect was abolished by adding phosphatidylserine/phosphatidylcholine liposomes to the aPTT system, and could be significantly reduced by a monoclonal antibody against a cross-reacting idiotype of antiphospholipid antibodies (MoAb 5.2) previously incubated with aCL. These results suggest that the anticoagulant properties of aCL are mediated by an interaction with phospholipid components of the coagulation system. The functional block of this activity using monoclonal MoAb 5.2 antibody could be useful for investigating the mechanisms by which aCL is associated with clinical thrombosis.