We have developed a PCR assay named ASPA (allele-specific primer amplification) to determine the HPA-5a and -5b genotypes. It consists of two PCR-reactions. One primer of each primer set has a 3'-end nucleotide which is specific for A or G at position 505 of the HPA-5b or -5a allele respectively. The HPA-5 genotypes determined in this way were strictly concordant with the genotypes established by the PCR-ASRA and with the phenotypes established using MAIPA. The ASPA is a rapid and reliable technique and can be used for the determination of alleles which code for platelet antigen allotypes.