AbstractTo study the idiotypes (Id) of the humoral response to H‐2Ldantigens, xenogeneic antisera were produced to three independent monoclonal antibodies, recognizing Lddeterminants H‐2.64 or 65. The reactivity of each of these anti‐Id was specific for the immunizing antibody, no cross‐reactions among the three anti‐L hybridomas being detectable. Antisera produced in BALB/c H‐2dm2mice hyperimmunized with BALB/c tissue were examined for the presence of cross‐reactive Id (IdCR). When the ability of these anti‐Id to inhibit the binding of alloantibodies to Ldsplenic antigens was tested, a broadly shared Id was detected by one anti‐Id (anti‐23‐10‐1S). The shared Id represented between one‐fourth and one‐half of the total anti‐Ldhumoral response and was found in every BALB/c H‐2dm2anti‐BALB/c serum tested. The IdCRwas limited to those alloantibodies reacting with the determinant H‐2.65 which corresponds with the serologic reactivity of monoclonal antibody 23‐10‐1S. The other two anti‐Id did not detect components of the anti‐Ldhumoral response, even though one of these was made against monoclonal antibody 30‐5‐7S which also detects serologic specificity H‐2.65. The detection of a IdCRin anti‐Ldresponses stands in contrast to our previous failure to detect IdCRin anti‐H‐2Kkresponses. Among the possible reasons for this contrast are (a) the fact that 23‐10‐1S is an IgM antibody, whereas the anti‐H‐2Kkantibodies studies were IgG; (b) the chance occurrence of anti‐H‐2 hybridomas with dominant Id and (c) the possibility that the anti‐Ldantibody