The monoclonal antibody immobilization of platelet antigens (MAIPA) assay has proved to be a reliable and useful tool in platelet immunology. After immunochemical localization of the most important platelet allo- and autoantigens on platelet glycoproteins (GPs) IIb/IIIa, Ia/IIa, Ib/IX, the corresponding antibodies can easily be detected with this technique because mAbs against these membrane constituents are now readily available. The assay also allows reliable typing of platelet alloantigens. Detection of GP-specific PAIgG (by direct MAIPA) has proved to be a specific diagnostic sign of AITP. The MAIPA assay allowed identification of 'new' alloantibody specificities including alloantibodies against 'low-frequency' antigens. The principle of this technique has successfully been applied to characterize antibodies against lymphocytes, endothelial cells and granulocytes.