Glycoprotein Ibalpha (GP Ibα; CD 42b; hereafter GPIBA) is a component of the cell surface receptor for the von Willebrand factor (vWf) on platelets. Immunizations against various platelet surface antigens play a major role in neonatal alloimmune thrombocytopenia and in post-transfusion purpura. Only one antigenic polymorphism in GPIBA has thus far been established: the HPA-2 (Ko) alloantigen system. To screen other polymorphisms in GPIBA systematically, we analyzed the whole coding sequence of theGPIBA gene in 50 Finnish blood donors using the single-strand conformation polymorphism method. In addition to the known polymorphisms, we detected three others. Sequencing of the gene segments carrying the new polymorphisms revealed that none of them changed the predicted amino acid sequence. Polymorphism designatedRS was located five base pairs upstream from the initiation codon at position 3064 and had the gene frequency of 16% forR and 84% forS, respectively, in the Finnish population, and it was detectable by the restriction enzymeHae III. TheEF polymorphism was at position 3842 (Asn242) and the gene frequencies were 97% forE and 3% forF. TheKL polymorphism was at position 4142 (Arg342) and the gene frequencies were 98% forK and 2% forL. The five polymorphic positions in GPIBA formed altogether six different alleles of the gene. The data suggest that there are only a few variable amino acids in GPIBA.