The human alloantigen system Bak a / b is associated with a Ile 8 4 3 -> Ser replacement on platelet glycoprotein IIb, the α-subunit of the integrin receptor for fibrinogen (GPIIb/IIIa). Recent immunological studies indicate that sialylated oligosaccharide chain(s) are also implicated in expression of the Bak a determinant. Here we show that the GPIIb fragment 704-856 contains the whole Bak a epitope, and that chemical cleavage of a single O-linked oligosaccharide chain within this GPIIb domain correlates with the loss of its anti-Bak a antibodies binding ability. Ser 8 4 7 was identified as the O-glycosylation site. Therefore, our results show that the Ser 8 4 7 modification is responsible for the expression of the GPIIb-specific Bak a alloantigen, and provide thus a link between the molecular biology and the immunologie observations.