Glanzmann's thrombasthenia (GT) is a recessive autosomal bleeding disorder characterized by the abnormality of aggregation due to a platelet glycoprotein (GP) IIb-IIIa deficiency or a dysfunctional complex. Molecular abnormalities have been localized on the gene coding for GP IIb or IIIa. The aim of our work was an attempt to obtain indirectly information on the putative localization of the molecular defect in patients with GT type I or II by the determination of the HPA-1 (GP IIIa) and HPA-3 (GP IIb) alloantigenic systems' expression in GT carriers. If GT results from a defective GP IIb gene, a GT carrier would appear homozygous for HPA-3 by serology, because the normal gene product will be expressed while the abnormal GP IIb gene product will not be present. Conversely, if the abnormality is in the GP IIIa gene, such an individual would appear homozygous for HPA-1. Therefore, the heterozygous status for HPA would result from the normal expression of the two genes for the considered alloantigenic system. Among the four families studied with informative members, our presumptions were strengthened by the preliminary genetic results in one family showing a mutation in the GP IIb gene. Thus, serology could be a simple screening test for the possible defective gene responsible for GT allowing molecular investigation focusing only on GP IIb or IIIa gene.