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We have studied the specificity of antiphospholipid antibodies in 148 patients with autoimmune diseases, 120 patients with systemic lupus erythematosus and 28 with the primary antiphospholipid syndrome. In addition, 20 patients suffering from syphilis were studied. As a control group, 64 healthy volunteers were investigated. Patient and control serum samples were tested for binding to seven different phospholipid antigens by ELISA. Interestingly, 90% of the sera from syphilis patients and 6% of the autoimmune patients exhibited a significant binding to platelet-activating factor (PAF), a molecule similar to the structure of phosphatidylcholine. In addition, the IgG fraction from one of the lupus patients, which showed a high binding activity to PAF, was further affinity-purified using both liposomes and an affinity chromatography column. Preincubation of these antibodies with PAF inhibited subsequent binding to immobilized PAF. These observations might suggest a putative interaction of antiphospholipid autoantibodies with PAF 'in vivo', which may have, in some patients, important pathophysiological consequences.