We have sequenced a cDNA clone encoding an HLA-B71 (B70) allele from an African American individual. Serologic definition of B70 allelic products is very difficult due to extensive cross-reactivity of the alloantisera with B35, B15, and B5 antigens. The new sequence most closely resembles the sequence of B*1503, differing only by three amino acids at positions 63, 67, and 116. The B71 sequence differs from alleles of the B15 antigenic group (serologically defined as B62) by 7-8 amino acids and from members of the B35 family by 10 to 12 amino acids. B71 may represent an evolutionary intermediate, sharing elements common to both B35 and B15 allelic groups.