Residue 69 of the DP beta chain has been previously identified as being involved in T-cell recognition as well as in the susceptibility to certain autoimmune diseases. The codon for Glu 69 in the DPB1*02012 allele was changed to the codon for Lys found in DPB1*0402, and transfectant L cells expressing wild-type or mutant HLA-DP molecule were obtained. The binding of a large panel of mAbs to these transfectants was tested by flow cytometry. Glu to Lys 69 substitution decreased the binding to the DPB1*02012 allele of some of the DP mAbs and completely eliminated the binding of four of the antibodies tested. These results clearly showed that this residue is involved in the formation of DP antibody-binding epitopes. Because this residue should be located in the alpha-helix of the DP beta polypeptide with the side chain pointing into the peptide-binding groove, its implication in the definition in some DP antibody-binding epitopes should be (a) defining conformational epitopes through effects on the conformation of adjacent regions of the molecule, and (b) determining the binding of peptides to the DP cleft which is directly or indirectly involved in these epitopes.