The platelet membrane glycoprotein (GP)Ib/IX complex is composed of three polypeptides, GPIb alpha, GPIb beta, and GPIX, and functions as a platelet receptor for von Willebrand factor. All three subunits are reported to be requisite for efficient surface expression of the complex. The absence of the GPIb/IX complex on platelet membrane is the hallmark of a congenital qualitative platelet disorder, termed the Bernard-Soulier syndrome (BSS). We describe here the molecular basis of a novel variant phenotype of BSS in a female patient, designated as BSS Kagoshima. Her platelets completely lacked the surface expression of GPIb alpha, but expressed a residual amount of GPIb beta and GPIX. Unexpectedly, her platelets and plasma contained a truncated GPIb alpha polypeptide with an apparent molecular weight of 116 kD (under nonreducing conditions). The amounts of truncated protein were 23% and 60% of the normal values in platelets and plasma, respectively. The abnormal protein contained a normal amount of sialic acid as demonstrated by digestion with neuraminidase. DNA sequencing analysis showed a homozygous single nucleotide substitution from the serine codon (TCA) to a nonsense codon (TAA) at residue 444 in the GPIb alpha gene. The mutant gene generated a truncated GPIb alpha molecule lacking the transmembrane region and cytoplasmic tail. Her parents were heterozygotes for the mutation. These findings suggest that this type of truncated GPIb alpha was produced, normally glycosylated, and subsequently secreted into the plasma. Furthermore, the truncated GPIb alpha might be associated with the process of the surface expression of incomplete GPIb/IX complex, GPIb beta and GPIX.