The HLA-DQ(alpha 1*0501, beta 1*0201) and-DQ(alpha 1*0501, beta 1*0202) (i.e., DQ2) heterodimers are probably involved in the pathogenesis of celiac disease and several other HLA-DQ-associated diseases. To obtain a tool for studies of these molecules, a mAb of the IgG1 isotype, 2.12.E11, was produced by immunization with purified DQ(alpha 1*0501, beta 1*0201) molecules and murine NIH 3T3 cells transfected with both DQA1*05011 and DQB1*0202. Panel cell studies with HLA homozygous B-lymphoblastoid cells and HLA-transfected murine cells demonstrated that 2.12.E11 bound only to cells expressing HLA-DQ beta 1*0201 or 0202, irrespective of the accompanying DQ alpha chain (i.e., DQ alpha 1*0501 or DQ alpha 1*0201). Another DQ2-specific mAb (XIII 358.4) and the broadly HLA class-II-reactive mAb Tü39 strongly inhibited binding of 2.12.E11. Epitope mapping employing mutants with single aa substitutions of DQ beta 1*0202 indicated that position 37 may be of some importance for 2.12.E11 binding. A triple mutant (45G-->E, 46E-->V, 47F-->Y) failed to bind 2.12.E11, suggesting a crucial role for one or more of these residues in the epitope. However, the expression of the mutant beta chain could not be formally proved, as none of the DQ2-reactive mAbs recognized this transfectant.