A total of 25,000 antisera from parous women was evaluated for antibody specificity toward HLA epitopes in the A,B, and C loci. It was determined that most of the reactivity patterns against a large panel could be accounted for by the amino acid substitutions at the known residues. It has been surprising that each of the variable amino acids at each residue against which antisera have been found belonged on either the A, B, or C locus.