HLA-B15 embraces a multiplicity of antigenic specificities which vary in their distribution amongst human populations. To correlate B15 molecular structure with the serological picture we have sequenced alleles encoding the various subspecificities of the B15 antigen: B62, B63, B75, B76 and B77, and a number of "variants" of these antigens including the 8w66 split of B63. HLA-B63 (B*1517) and 8w66 (B*1516) heavy chains have sequence identity to B17 in the alpha 1 helix correlating with the antigenic crossreactivity of these molecules. HLA-B77(B*1513) and B75 (B*1502) heavy chains differ solely in segments determining the Bw4 and Bw6 public epitopes, consistent with the serological description of the B77 and B75 antigens. One allele encoding the B76 antigen (B*1512) appears to be the product of gene conversion between the HLA-A and -B loci and differs from B*1501 in codons 166 and 167. In contrast, a second allele encoding the B76 antigen (B*1514) differs from B*1501 by an unrelated substitution in codon 167 which confers similarily with B45, an antigen crossreactive with B76. A third allele encoding B76, B*1519, differs from B*1512 by a unique point substitution in exon 4. Three alleles encoding variant B15 and B62 antigens (B*1508, B*1511 and B*1515) differ from B*1501 by localized clusters of substitutions that probably result from interallelic conversion. The B15 sequences described in this paper, in combination with those previously determined, define a family of 22 alleles, including those encoding the B46 and B70 antigens. Within this family the patterns of allelic substitution are analogous to those of other HLA-A and -B families, in that pairwise differences almost always involve functional positions of the antigen recognition site and recombination is the major agent of diversification.