The ABPC-48 Id (A48 Id) is normally not expressed in detectable amounts in the serum of BALB/c mice that have been immunized with bacterial levan (BL). However, A48 Id-bearing anti-BL clones can be activated in BL-immunized mice by three distinct prior treatments: (a) administration of A48 Id-bearing monoclonal proteins to newborn mice; (b) administration of minute amounts of anti-Id antibodies to newborn mice; and (c) production of anti-(anti-A48 Id) antibodies (Ab3), in adult mice. From these three groups of mice, eighteen monoclonal antibodies (MAb) expressing A48 Id were obtained. Regarding the binding specificity, these MAb can be divided into three groups: one that binds only BL, the second that binds BL and displays low cross-reactivity for inulin, and the third that lacks BL- and inulin-binding activity. This latter group was obtained only from adult mice immunized with anti-A48 Id-KLH conjugate. Immuno-chemical analysis of these MAb has shown that the A48 Id is made up of several idiotopes, some of them associated with the combining site and others nonantigen inhibitable. Comparisons of the amino acid sequence of the UPC-10 and A48 VH regions, and the distribution of the A48 Id family on A48, UPC-10, and three MAb, suggested that A48 regulatory idiotypes can be located on the framework segment of VH region. Furthermore, we screened 198 mouse and 80 human myeloma proteins for their ability to express A48 Id. Of these, only MOPC-167, an IgAk phosphocholine (PC)-binding myeloma protein, gave a significant inhibition of binding of labeled A48 to anti-A48 Id antibodies by radioimmunoassay and enzyme-linked immunosorbent assay. In addition, the binding of labeled MOPC-167 to anti-A48 Id antibodies was not inhibited by PC but was inhibited by A48 and 3-76-42 MAb bearing A48, UPC-10 non-antigen-inhibitable idiotopes. These results extend our prediction that the regulatory idiotopes can be expressed not only on antibodies specific for a family of antigens or members of the same network pathway, but also can be shared by antibodies with different antigenic specificity.