Amino acid substitutions in platelet membrane glycoproteins result in alloantigens. Identifying these polymorphisms is important in alloimmune-mediated platelet disorders. Immunophenotyping platelet antigens can be limited by the unavailability of specific antisera. The goal of this work was to identify human platelet antigen genotypes in individuals using a technique that would circumvent the limitations of immunophenotyping and be clinically applicable. We have successfully applied the reverse dot-blot (RDB) technique to the genotyping of the five major human platelet alloantigen systems. Allele-specific oligonucleotides (ASOs) representing each allele of these alloantigens were covalently linked to a filter. Biotinylated oligonucleotides flanking the polymorphic sequences in genomic DNA were used to amplify genomic DNA by the polymerase chain reaction (PCR), and these products were hybridized to the filters containing the ASOs. Reactivity was detected with a chromogenic substrate. This nonradioactive methodology identifies all 15 possible genotypes in a well-defined control group of individuals and requires only two PCR reactions per patient sample. RDB analysis was used to successfully genotype women and family members with neonatal alloimmune thrombocytopenia and with posttransfusion purpura and to prenatally genotype the amniocytes from a fetus at risk for thrombocytopenia. The RDB methodology is specific, sensitive, and rapid and should enhance our ability to accurately diagnose disorders of alloimmune platelet destruction.