AIMS--To determine whether beta 2 glycoprotein I (beta 2GPI) dependent anticardiolipin (aCL) antibodies detected in solid phase enzyme linked immunosorbent assays can also have lupus anticoagulant activity. METHODS--Six anticardiolipin antibodies were affinity purified from patients with these antibodies and lupus anticoagulant activity in their plasma. RESULTS--The anticardiolipin antibodies bound only to anionic phospholipids in the presence of beta 2GPI and bound to beta 2GPI in the absence of phospholipids. Four out of six had lupus anticoagulant activity in the dilute Russell viper venom time test. CONCLUSIONS--The results show that some beta 2GPI dependent aCL are lupus anticoagulants. It is unclear why only some should have lupus anticoagulant activity while others do not.