Antibodies raised in SWR/J mice (H-2q, Igc) to the random copolymer poly(LGlu60, LPhe40) (GPhe) were purified by immunoadsorbent chromatography and used to immunize a New Zealand red rabbit. The rabbit anti-idiotypic antiserum thus produced strongly inhibited the binding of 125I-GPhe by anti-GPhe antisera produced only in mice of H-2q haplotype, and had no effect on the binding of GPhe by anti-GPhe antisera produced in mice of other haplotypes, namely, H-2k and H-2p. The anti-idiotypic antiserum also inhibited the binding of GPhe by anti-GPhe-methylated bovine serum albumin antisera produced only in mice of H-2q haplotype. No linkage to Ig allotype was observed. The anti-GPhe antisera produced in F1 mice the anti-idiotypic antiserum demonstrating the dominant presence in these F1 mice of idiotypic determinants whose expression is dictated by the H-2q major histocompatibility complex (MHC). The anti-idiotypic antiserum also inhibited the binding of 125I-poly)LGlu56, LLys35, LPhe9) and 125I-GPhe antisera produced only in mice of H-2q haplotype. These specificities were also confirmed by the inhibition of the plaque-forming cells. It was concluded that the antibodies produced in mice of H-2q haplotype against GPhe and GLPhe share common idiotypic determinants that are recognized by the anti-idiotypic antiserum. A possible explanation for the unique findings that the expression of anti-GPhe idiotypic determinants in mice of H-2q haplotype are dictated by the gene product in the MHC is that the macrophages in mice of H-2q haplotype present unique determinants of GPhe polymer in the response process to GHphe.