The presence of anti‐CD36 antibodies in plasma of patients with thrombotic thrombocytopenic purpura (TTP), idiopathic thrombocytopenic purpura (ITP), and heparin‐induced thrombocytopenia without/with thrombosis (HIT/HITT) has been examined by immunoblots, and a monoclonal antibody capture assay, the platelet‐associated IgG characterization assay (PAICA). Results with PAICA showed that 73% (8/11) of patients with TTP were positive, and 71% (10/14) by immunoblots. With ITP, 20% (6/30) were positive by PAICA and 19% (3/16) by immunoblots; HIT, 30% (3/10) were positive by PAICA and 60% (6/10) by immunoblot; HITT, 50% (2/4) by PAICA and 100% (4/4) by immunoblot. Purification of CD36 by fast protein liquid chromatography (FPLC) from Triton X‐100 extracts of normal platelet membranes resulted in the isolation of two different forms: the classic 88 kD form, and a second, lighter 85 kD form. Our data indicated that the patients' plasma autoantibodies reacted strongly with the 85 kD form. Conventional monoclonal and polyclonal antisera produced to the 88 kD form reacted strongly with the 88 kD form but weakly with the 85 kD form. These results confirm the possible importance of anti‐CD36 antibodies in the pathophysiology of TTP and other thrombocytopenias and demonstrate the presence of a previously unrecognized target antigen for these antibodies.