The authors have developed a sensitive immunoenzymatic method for assaying anti-cardiolipin antibodies in the serum of patients with lupus (SLE). These antibodies were present in the serum of 43/108 SLE patients, particularly in those patients with either false syphilis serology (p less than 0.02) or circulating anticoagulant (p less than 0.05). The mean titre of anti-cardiolipin antibodies was higher in the group with positive VDRL (less than 0.03). The anti-cardiolipin antibody titre was independent of the anti-native DNA antibody titre, but there was a correlation with the anti-denatured DNA antibody titre (p less than 0.02). This correlation can be partially explained by the antigenic similarity (phosphodiester bridge) between the two molecules. The preliminary clinical studies have not shown any correlation between the presence of anti-cardiolipin antibodies and the presence of signs such as thrombocytopenia, haemolysis, cerebral vascular accident, venous thrombosis, recurrent abortion. A longitudinal study of certain patients suggests that the anti-cardiolipin antibodies may disappear at the time of thrombotic accidents, which induces fixation of these antibodies to a platelet or vascular target and as a result of corticosteroid therapy.