Abstract It has been previously shown that it is possible to modify the expressed repertoire of a given individual using idiotypic manipulation. For example, A/J mice respond to arsonate challenge by synthesizing a dominant idiotype, CRI A, whereas BALB/c mice do not. However, after treatment with rabbit polyclonal anti-CRI A antibodies (Ab2 or anti-idiotypic antibodies) and arsonate, BALB/c mice are able to synthesize a CRI A-like idiotype. To determine whether this modification of repertoire is dependent on the immunoglobulin loci (Ig-h, κ), we have analyzed the anti-arsonate response after anti-idiotypic treatment of three strains of mice (C58, C.C58, AKR), chosen because they are among a small group of trains which express Kappa V regions not seen in other strains. There are also L chains lacking in these strains which are expressed in other mice. The C58 and C.C58 strains share the same Ig-h locus ( Ig- h a ) with BALB/c mice but C.C58 are congenic mice, that express the κ loci on a BALB/c genetic background. AKR mice express the Ig- h d haplotype. AKR, C58 and C.C58 do not produce CRI A positive antibodies in response to arsonate; a defect which has been previously mapped to the κ locus. These three strains of mice (C58, C.C58 and AKR) were treated with rabbit anti-CRI A and boosted with Ars-KLH. The results show that after such treatment, the C.C58 mice were able to express CRI A-like antibodies which are serologically identical to those of BALB/c.