HLA class I-transgenic mice express HLA antigen on their tissues and establish self-tolerance to the expressed monomorphic and polymorphic determinants. When challenged with skin grafts and lymphoid cells of a second HLA-transgenic mouse expressing a different HLA molecule, a specific immune response is elicited that is focused on the determinants specified by the allelic HLA differences between donor and recipient transgenic mice. In the studies described, this has led to the production of a number of monoclonal antibodies with specificity for the HLA-Cw3 and HLA-B7 crossreactive groups of class I antigens. These results indicate that immunization of appropriate transgenic strains of mice with murine cells expressing a different HLA allelic transgene should permit the generation of monoclonal antibodies of narrow specificity against virtually any polymorphic epitope on HLA antigens.