Public idiotypes (IdX) expressed on monoclonal antibodies (mAb) against a monomorphic alpha-chain determinant of the I-E molecule (Ia.7 epitope cluster I) have been studied by using xenogeneic anti-Id reagents derived from pig, rabbit, and rat. IdX+ anti-Ia.7 mAb were recently demonstrated to be structurally related by a high frequency expression of the V kappa 21E light chain subgroup. This raised the question of whether V region determinants of the IdX were related to V kappa 21E sequences or whether they were unique to hypervariable regions of Ia.7 binding antibodies. To clarify this question, the possible association between the expression of the public Id (IdX(s)Ia.7) and the presence of V kappa sequences (V kappa 21E and/or J kappa segment) was examined. The reactivity of the anti-Id reagents with a random panel of 28 myeloma products (each containing a light chain from one of the different V kappa 21 subgroups) was studied by assaying the ability of these mAb to inhibit the binding between the anti-Id and anti-Ia.7 mAb. This analysis demonstrates that what has previously been defined as IdX Ia.7 includes determinants shared by V kappa 21E and V kappa 21D light chain V regions. The structures recognized are expressed irrespective of the J kappa segment. In addition, this study demonstrates interspecies variation in immune responses to such V kappa 21E antigenic determinants. Additional IdX components are found on anti-Ia.7 mAb but not on other V kappa 21E or D proteins. Thus V region subgroup considerations have crucial implications for Id characterization. In addition, this work describes the first division of the V kappa 21 subgroup into component parts by a mAb.