The autoimmune disease known as autoimmune thrombocytopenic purpura (ITP) is clinically defined by a low numbers of platelets in the circulation blood. Anti-platelet antibodies bind to glycoprotein molecules on the membranes of platelets and result in their dysfunction and destruction. Despite a growing body of information about ITP, it is difficult to isolate and characterise anti-platelet antibodies, because only limited monoclonal antibodies are available from ITP patients. This study used a phage display system to recognise Fab anti-platelet antibodies. Anti-platelet Fab-expressing phage was isolated by sequential panning of an ITP Fab library against normal non-ITP platelets. After isolation, the anti-platelet Fab-expressing phage was characterised by ELISA and Western blotting. The Fab-bearing phage pool obtained from five rounds of panning was analysed in order to determine its anti-platelet reactivity. Of the phage colonies obtained, 100 colonies of different sizes were randomly selected for reaction with whole platelets, using Ml3 phage as a negative control. 12 colonies of them had strong reactions against the whole platelet preparation, but only four colonies showed substantial reactivity against the lysed platelet preparation (lysate). Colony S7 showed highest the greatest degree of binding to both the lysate and the whole platelet preparation. The specificity of the four colonies (S2, S7, S8 and S9) that had strong positive reactions against platelet antigens was determined for the glycoprotein component GP Ilb/IIIa. Further characterisation of the proteins in the lysate preparation was carried out using blotting techniques. The protein content of the four Fab-bearing phage colonies was quantified under the non-reducing conditions of Western blotting to evaluate their ability to recognise platelet antigens. Three of the four colonies showed three bands representing proteins with different molecular weights. Each of these three colonies had one band that corresponded to a protein of molecular weight 92 kD. The fourth colony showed only a single band, but this band also corresponded to a 92-kD protein.