We previously characterized major (IdX Ia.7) and minor (IdI) idiotopes in a collection of monoclonal alloantibodies reactive with monomorphic (i.e., Ia.7-like) determinants in the structural domain I of the murine class II I-E molecules. In this report, preliminary structural characterization of this antibody family is presented. First, the contribution of isolated H and L chains of the anti-Ia.7 cluster I mAb 41.A to IdX Ia.7 and IdI 41.A idiotope expression was evaluated by testing the capacity of these chains, either isolated or reassociated in homologous or heterologous hybrid Ig, to inhibit the binding of rat or mouse anti-idiotope mAb to IdX Ia.7+ mAb coated plates. It was found that the IdI 41.A idiotope defined by the mouse anti-idiotopic mAb H90-21.1 required the presence of both 41.A H and L chains for complete expression, while the rat mAb-defined IdX Ia.7 idiotope could be detected on isolated and on reassociated 41.A L chain. To evaluate further the structural correlates of the IdX Ia.7 idiotope, H, L, or both H and L chains of 5 A.BY, 4 A.TH and 1 C3H.SW IdX+ anti-Ia.7 mAb, as well as that of 3 A.TH IdX- anti-I-E or anti-I-A and -I-E mAb were subjected to NH2-terminal amino acid sequencing. These analyses demonstrated a) that different H chains corresponding to different subgroups (at least to the VHII and VHIII) could be expressed without apparent modification of IdX Ia.7 idiotope expression and b) that 9 of 11 IdX+ anti-Ia.7 mAb utilized highly homologous L chains of the VK21E subgroup. The relevance of these findings to the genetic control of the idiotypic markers identified in the Ia.7 system is discussed.