In spite of the potential role of antiidiotypic (anti-id) antibodies in the immune response to mismatched HLA antigens and in the outcome of renal allografts, the idiotypic cascade in the HLA system has been characterized to a limited extent. For instance, it is not known whether anti-id antibodies defining idiotopes coexpressed in the combining site of the original anti-HLA antibody selectively stimulate distinct subsets of B cell clones. Since this information contributes to our understanding of the role of anti-id antibodies in the generation of diversity in anti-HLA immune response, we have determined whether a preferential recognition of the immunizing anti-id mAb is displayed by the two subsets of anti-HLA-DR mAb elicited with anti-id mAb F5-444 and F5-830. The latter two mAb recognize idiotopes coexpressed in the antigen-combining site of the immunizing anti-HLA-DR1,4,w14,w8,9 mAb AC1.59. All the anti-HLA-DR mAb elicited with mAb F5-830 displayed a higher functional affinity for the immunizing anti-id mAb than the anti-HLA-DR mAb elicited with mAb F5-444. This finding reflects the higher reactivity of the subset of anti-HLA-DR mAb elicited with mAb F5-830 with the light chain of the immunizing mAb. The present study, therefore, shows for the first time that distinct anti-id mAb recognizing idiotopes coexpressed in the combining site of anti-HLA-DR mAb stimulate different subsets of idiotope positive B cell clones in a nonrandom fashion. This preferential selection by anti-id mAb affects the characteristics of the immune response, as the two subsets of anti-HLA-DR mAb display differences in their fine specificity. These results are consistent with the possibility that anti-id antibodies may play a role in the changes in the specificity of anti-HLA antibodies that may occur in the course of an immune response to mismatched HLA alloantigens.