Glanzmann's thrombasthenia is a rare hemorrhagic syndrome, characterized by a quantitative or functional defect of the platelet glycoprotein GPIIb-IIIa complex. The authors describe a method to diagnose thrombasthenic patients and identify carrier subjects by using monoclonal antibodies specific for GPIIb and GPIIIa in an immunoblotting technique. The immunoreaction patterns of two thrombasthenic patients lacking GPIIb or GPIIIa, respectively, are shown. The described method produces further evidence concerning the biochemical heterogeneity of Glanzmann's thrombasthenia.