Bernard-Soulier syndrome (BSS) giant platelets have defective and/or deficient glycoprotein (GP) Ib/IX complexes, causing absent ristocetin- induced aggregation, defective interaction with von Willebrand factor, morphologic abnormality, and a clinical bleeding tendency. Recently several mutations have been described in the platelet GPIb alpha gene in individuals exhibiting the BSS phenotype. We have studied a family with classical BSS, and have excluded lesions at the GPIb alpha locus by restriction fragment length polymorphism linkage analysis. Analysis of the genes for two other components of the platelet GPIb:IX complex, namely GPIb beta and GPIX, showed two different missense mutations in the coding region of the GPIX gene: an A-->G transition in codon 21 results in conversion of an aspartic acid to glycine and an A-->G change in codon 45 converts an asparagine residue to serine. Three affected individuals are doubly heterozygous for these mutations, which alter conserved residues in or flanking the GPIX leucine-rich glycoprotein motif. Both mutations create new recognition sites for the enzyme Fnu 4H1; therefore, this enzyme was used to screen 60 normal subjects (120 alleles). Neither mutation was detected in any subject other than direct relatives of the affected individuals. Although low levels of GPIb were demonstrable by both flow cytometry and immunoblot analysis in an affected individual's platelets, there was no evidence of GPIX immunoreactivity. We propose that expression of abnormal GPIX prevents stable assembly of the GPIb/IX complex, causing BSS in the doubly heterozygous individuals in this family.