GP Ia/IIa (also called VLA-2 or alpha 2 beta 1) is the primary receptor for collagen on platelets. The human platelet alloantigens HPA-5a(Brb) and HPA-5b(Bra) have been found to reside on the platelet GP Ia/IIa complex. In order to establish the molecular basis of the HPA-5 system, platelet RNA was isolated form HPA-5 (a+, b-) and HPA-5(a-, b+) individuals. After reverse transcription, cDNA coding for glycoprotein Ia (GP Ia) was amplified by the polymerase chain reaction (PCR). Nucleotide sequence analysis of the PCR products revealed an A-->G polymorphism at base pair 1648 of the coding region of the mature protein, resulting in a substitution of lysine (AAG) in HPA-5b(Bra) by glutamic acid (GAG) in HPA-5a(Brb) at amino acid 505. Subsequent PCR-ASRA (allele-specific restriction enzyme analysis) with Mnl I using cDNA derived from three HPA-5 (a+, b-), one HPA-5 (a+, b+) individuals demonstrated that HPA-5a and -5b alleles are distinguishable by DNA typing. In addition to the A-->G substitution at base pair 1648, three silent mutations were identified, G-->C (195 bp), C-->T (837 bp), G-->A (1041 bp).