The simultaneous occurrence of platelet antibodies (ab) and a circulating "antithromboplastic" anticoagulant in a patient with thrombocytopenia led to the hypothesis that antiphospholipid ab may be causing both phenomena. Of 55 sera obtained from thrombocytopenic patients exhibiting a positive microtest for complement fixation (CFT) with platelets, 37 also gave positive results with highly purified phospholipids (Phl) used as antigen. In order to further evaluate the role of Phl, 40 different liposome suspensions obtained by sonication of various mixtures of Phl (with/without addition of cholesterol) were tested as antigen in the CFT with 11 selected sera, and as platelet factor 3 (PF3) reagent in the partial thromboplastin time test with normal plasma. Eight liposome preparations with PF3 activity (all containing phosphatidyl-serine) were equally active as antigen (ag) in the CFT. Liposomes composed of phosphatidyl-ethanolamine and sphingomyeline delivered ag-activity only. Since the two substances are accessible components of the outer membrane surface of thrombocytes, anti-Phl-ab may well bind to platelets in vivo, causing thrombocytopenia. Coagulation-inhibiting activity of these ab could be directly demonstrated in a PF3-test system (thrombocytopathy caused by PF3 inhibition). The identity of antithromboplastic anticoagulant and anti-Phl-ab was further substantiated by immunoabsorption, since both activities were simultaneously eliminated from the sera with a Phl-charcoal adsorbent.