The frequency of each A*10 allele was determined in 5 major United States ethnic populations randomly selected from a pool containing 82,979 unrelated individuals. The phenotype frequency of A10 was 10.5% in Caucasians, 14.0% in African-Americans, 21.1% in Asians/Pacific Islanders, 10.6% in Hispanics, and 9.8% in Native Americans. Fifty-nine individuals who had at least one A10 antigen were randomly chosen from each ethnic group for polymerase chain reaction using sequence-specific oligonucleotide probes (PCR-SSOP) typing. Thirteen of sixteen known A10 alleles were identified in this pool. The most common alleles observed were: A*2601 in Caucasians (55%), Hispanics (58%), and Native Americans (45%); A*3402 in African-Americans (34%); and A*3401 in Asians/Pacific Islanders (61%). The African-American and Asian/Pacific Islander populations differ from all other populations in the distribution of A*10 alleles, particularly, A*2601, A*3401, and A*3402.