The humoral immune response in strain A mice to protein conjugates of p-azophenylarsonate (Ars) is characterized by the presence of a major cross-reactive idiotype denoted as IdCR. Previous molecular analyses of monoclonal IdCR+ Ars-binding antibodies isolated from multiply immunized animals have indicated that these antibody variable (V) regions may be the expressed product of a single combination of VH, D, JH, V kappa, and J kappa gene segments. The basis of this apparent domination of the Ars response by V regions encoded by this single combination of gene segments is unclear, but is discussed in this report. Our structural analyses on five monoclonal IdCR+ antibodies that are unable to bind Ars show that in contrast to those of Ars-binding IdCR+ antibodies, these (Ars-nonbinding) IdCR+ V regions are encoded by multiple combinations of VH, D, JH, V kappa, and J kappa gene segments, but with the commonality that they all utilize a single VH gene segment (VHIdCR). We provide examples in which the VHIdCR gene segment is expressed with three different V kappa gene segments and with each of the four JH gene segments to produce serologically detectable IdCR+ Ars-nonbinding antibodies. It would thus appear that the previous failure to detect alternative IdCR+ V segment combinations was due to a sampling procedure requiring that the IdCR+ antibody bind Ars, and not the result of restricted assembly or expression of the VHIdCR gene segment with a particular combination of D, JH, V kappa, and J kappa gene segments. This bias in protocol, however, cannot completely account for the homogeneity in previously studied IdCR+ Ars-binding antibodies, because we were able to isolate, from primary immune responses, IdCR+ antibodies that do bind Ars but that utilize alternative V segment combinations. This finding suggests that combinations of V gene segments encoding IdCR+ antibodies are more numerous in primary as opposed to secondary immune responses, and raises the question of why a single combination of VH, D, JH, V kappa, and J kappa gene segments dominates the secondary strain A immune response to Ars.