We have previously described an experimental system for studying the cellular and molecular basis of idiotype expression (1-5). BALB/c mice produce antidinitrophenyl (DNP) ~ antibody expressing a recurrent idiotypic determinant, Id-460. Two genes controlling the expression of this determinant map to the heavy and kappa chain variable regions (VH and V,j). Thus, the determinant is likely to be a combinatorial determinant involving both VH and V,l domains. 1d-460 is also found in normal serum, where much of it binds Pasteurella pneumotropica (P. pneumotropica), and none to DNP. Furthermore, P. pneumotropica immunization induces production of non-DNP-binding, Id-460 + antibodies. To understand the structural basis for this shared idiotypy, we have undertaken a molecular analysis of monoclonal Id-460 + antibodies. We have previously described the development of molecular probes for and the structure of the genes encoding the MOPC-460 (M460) protein, the prototypic DNP-binding antibody in the Id-460 system. We have also reported the structure of two Id-460 + hybridomas, LB8, which binds P. pneumotropica, and D35, which is DNP specific. These three proteins are encoded by closely related VH and V, gene segments, but differ in other gene segments (5). In the present experiments we use these probes to establish the molecular and genetic relatedness of other Id-460 + monoclonal antibodies in an attempt to determine the molecular basis of shared idiotypy by antibodies of differing antigen binding specificity.We have produced a panel of monoclonal antibodies selected for Id-460 expression. We obtained these Id-460 + monoclonal antibodies in two ways: by anti-Id-460 immunization or by DNP immunization. In this report, we demonstrate that all of the Id-460 ÷ hybridomas tested share immunoglobulin (lg) gene rearrangements and DNA sequence homologies for both Vu and VL with M460.