Decreased expression of functional IIbβ3 complexes on the platelet surface produces Glanzmann thrombasthenia. We have identified mutations of IIb^P145 in 3 ethnically distinct families affected by Glanzmann thrombasthenia. Affected Mennonite and Dutch patients were homozygous and doubly heterozygous, respectively, for a P¹⁴⁵A substitution, whereas a Chinese patient was doubly heterozygous for a P¹⁴⁵L substitution. The mutations affect expression levels of surface IIbβ3 receptors on their platelets, which was confirmed by co-transfection of IIb^P145A and β3 cDNA constructs in COS-1 cells. Each mutation also impaired the ability of IIbβ3 on affected platelets to interact with ligands. Moreover, when IIb^P145A and β3 were stably coexpressed in Chinese hamster ovary cells, IIbβ3 was readily detected on the cell surface, but the cells were unable to adhere to immobilized fibrinogen or to bind soluble fluorescein isothiocyanate–fibrinogen after IIbβ3 activation by the activating monoclonal antibody PT25-2. Nonetheless, incubating affected platelets with the peptide LSARLAF, which binds to IIb, induced PF4 secretion, indicating that the mutant IIbβ3 retained the ability to mediate outside-in signaling. These studies indicate that mutations involving IIb^P145 impair surface expression of IIbβ3 and that the IIb^P145A mutation abrogates ligand binding to the activated integrin. A comparative analysis of other IIb mutations with a similar phenotype suggests that these mutations may cluster into a single region on the surface of the IIb and may define a domain influencing ligand binding. (Blood. 2000;95:180188)