Glanzmann's thrombasthenia is a bleeding disorder characterized by a decrease or absence of the functional platelet membrane glycoprotein (GP) complex, GPIIb/IIIa (alpha IIb beta 3). We describe a new deletion-insertion mutation in the GPIIb gene causing type I Glanzmann's thrombasthenia in two siblings of a consanguineous Iranian-Jewish family. The proband's platelets bound more antibodies against the vitronectin receptor-alpha V beta 3 than normal platelets, suggesting a normal GPIIIa (beta 3) gene and a defect in the GPIIb gene. Sequencing of amplified cDNA and genomic DNA fragments showed a 6-bp deletion and 31-bp insertion in exon 25 of the GPIIb gene. The predominant platelet GPIIb mRNA of the proband was a product of the splicing of exon 24 to a cryptic AG acceptor site in the insertion and encoded for deletion of amino acids Leu817-Asn826 and insertion of eight different amino acids. Cotransfection of COS-7 cells with expression vectors containing wild-type GPIIIa cDNA and the mutated GPIIb cDNA failed to produce detectable amounts of GPIIb/IIIa on the surface of the cells. Allele-specific restriction analysis of genomic DNA of family members showed homozygosity for the mutation in the affected siblings, heterozygosity in the parents, and homozygosity for the normal allele in an unaffected sibling. The observed mutation is in a region that is conserved from rodents to humans and has been suggested to be involved in the interaction between GPIIb and GPIIIa when these GPs are complexed in solution.