A DNA-based one-stage technique, polymerase chain reaction with sequence-specific primers (PCR-SSP) was developed for genotyping of the platelet specific alloantigen HPA-5 (Bra/Brb). Sequence-specific primers, matching the wild type and the point mutation responsible for the HPA-5 (Bra/Brb) phenotype, were constructed. Conjointly a fragment of the gene coding for glycoprotein (GP) IIIa was amplified as an internal control of the enzyme reaction. Using these HPA-5 (Bra/Brb) sequence-specific primers the correct fragment of the GPIa gene was amplified, as evidenced by the PCR product size, the restriction map and by the nucleotide sequence. This assay was applied on 187 Swedish blood donors; 157 individuals were found to have a homozygous HPA-5a (Bra/Brb) genotype and 30 individuals a heterozygous HPA-5a,b (Bra/Brb) genotype. None of the donors was found to display a homozygous HPA-5b (Bra/Brb) genotype. Thus, the (HPA-5b) Bra antigen frequency in this population will be approximately 16.0% with a gene frequency of 8.0%. It is concluded that this assay is an attractive technique for genotyping of the HPA-5 (Bra/Brb) alloantigens on genomic DNA. The technique can replace serological alloantigen typing, especially in cases where platelets and rare human alloantisera are not available.