The HLA-A2-specific mouse monoclonal antibody BB7.2 plus complement has been used to immunoselect variant clones of the lymphoblastoid cell line T5-1 (HLA-A1, -A2, -B8, and -B27). Members of one class of variant clones appear to express cell surface HLA-A2 molecules that display reduced reactivity with the selecting antibody, but normal or near normal reactivities with some other HLA-A2-specific monoclonal antibodies and human alloantisera. The HLA-A2 heavy chains derived from two of these variant clones were characterized by comparative double-label tryptic peptide mapping in conjunction with microsequence analysis. These heavy chains were found to carry distinct mutations in the same peptide in the molecule. We conclude that residues within this short segment of the polypeptide contribute to an HLA-A2-specific serological determinant.