GPIIb/IIIa, the human platelet glycoprotein complex, is the autoantigen most commonly recognized by autoantibodies in autoimmune thrombocytopenic purpura (AITP). Two murine monoclonal antibodies (mAbs), namely Y2/51 and 5B12, directed against gpIIIa and gpIIb/IIIa, respectively, and rabbit anti-human platelet polyclonal antibodies have been used to select AITP-related epitopes from a phage display peptide library expressing random dodecapeptides in the pIII coat protein of M13 phage. The selected phage clones were tested by ELISA for binding to rabbit anti-human platelet antibodies as well as to sera from AITP patients. Seven clones reacted strongly with rabbit anti-human platelet antibodies, and four clones reacted with sera from AITP patients. Some homology between peptide inserts sequences of selected clones and human platelet gpIIIa and gpIb were found.