The authors describe a method for the diagnosis of heredital platelet membranopathies by means of monoclonal antibodies against the main membrane glycoproteins of thrombocytes, glycoprotein Ib and IIIa. The platelets are differentiated in the flow fluorocytometer from other blood cells by the typical optic profile caused by their size and granular character. Monoclonal antibodies are bound to the appropriate membrane glycoprotein and their amount is then detected by means of a secondary antibody labelled with fluorescein. The intensity of fluorescence of individual platelets is proportional to the number of molecules of the appropriate glycoprotein on their surface. By the above technique a case of Glanzmann's thrombasthenia was diagnosed, a rare hereditary haemorrhagic disease, characterized by the absence or abnormal function of glycoprotein complex IIb/IIIa the platelet receptor for fibrinogen.