Hybridization of a non-secreting mouse myeloma cell line NSO with splenocytes from a BALB/c mouse hyperimmunized with the anti-CD4 monoclonal antibody (mAb) HP2/6 generated the anti-idiotypic (anti-id) mAb F11-2113, F11-2302 and F11-2444, which recognize idiotope(s) (id) that are the same or spatially close to each other and are located outside the antigen-combining site of the immunizing mAb. Binding and inhibition assay showed that id are not expressed either on other mouse anti-CD4 mAb or polyclonal immunoglobulins (Ig). Western blotting analysis showed that the id defined by anti-id mAb F11-2113, F11-2302 and F11-2444 are similarly expressed on separated heavy and light chains of mAb HP2/6 and suggested they are likely to be 'sequence-dependent' since their expression is conserved following SDS and reducing reagents treatment. This finding is unique inasmuch as 'sequence-dependent' id similarly expressed on heavy and light chains have been described only on a mouse monoclonal auto-antibody with immunoregulatory properties.