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Background and Objectives: Platelet-specific alloantigens are important in neonatal alloimmune thrombocytopenia, posttransfusion purpura, refractoriness to platelet transfusions, and population genetics. Data are scarce on allele frequencies in ethnic groups other than whites and Asians., Materials and Methods: Using allele-specific restriction enzyme analysis, we studied the distribution of HPA-1 and HPA-2 alleles in six Brazilian Amazon tribes of Amerindians, belonging to five different language stocks. We compared these with the values obtained for blacks and whites., Results: Only the HPA-1a allele was found among 132 Amerindian chromosomes, compared with a gene frequency of HPA-1b of 0.115 and 0.113, respectively, among blacks and whites. The frequency of HPA-2b among the Amerindians (0.042) is lower than that obtained for blacks and whites (0.148 and 0.100, respectively), and the lowest thus far observed in a population of Asian origin., Conclusion: Differences in DNA polymorphisms in Amerindian populations have not only anthropological and genetic interest, but also practical applications when they involve coding regions that may change the functional or immunologic features of the protein.