Inherited giant platelet disorders (IGPD) are a heterogeneous group of rare diseases characterized by thrombocytopenia, large platelets and variable bleeding symptoms. Glycoprotein (GP) expression on platelet surface in these conditions is poorly characterized. We have investigated the expression of constitutively expressed platelet membrane GP and the response to TRAP activation in a group of patients with different forms of inherited macrothrombocytopenias.Two patients diagnosed Epstein syndrome (ES), two with Bernard-Soulier syndrome (BSS) and eight with Mediterranean macrothrombocytopenia (MM) were studied using flow cytometry combined to monoclonal antibodies (MoAbs). Mean platelet volume was also measured.In general, there was an increase in the binding of MoAbs directed against platelet membrane GPs in the patients studied (except, obviously, in BSS patients). The observed increase ranged between 0.9 and 3.36 times the value of the control for GPIbalpha, between 1.36 and 7.2 times for GPIIb-IIIa and 0.94 and 8.07 times for GPIV. However, when the observed value was divided by the measured platelet volume, the estimated density was similar to controls in the case of GPIbalpha but were increased for GPIIb-IIIa and GPIV. TRAP activation provoked significant increments in the number of copies of GPIIb-IIIa complexes present on platelet surface in controls (91%), MM patients (49%), but almost no changes in BSS platelets (6%).In the group of macrothrombocytopenia studied, an increase in the expression of platelet membrane GP was observed, although a wide variability exists even in patients with the same disorder.