Previous investigations have demonstrated close genetic linkage between loci governing expression of strain-specific idiotypes and immunoglobulin heavy (H) chain allotype (i.e., the C(H) locus). This linkage is presumed to reflect polymorphism of V(H) genes (or of their expression) linked to the polymorphic C(H) locus. That there was no apparent involvement of light (L) chain loci (thought to be unlinked to H chain) in inheritance of the idiotype-positive (Id(+)) phenotype was surprising, because the L chain is required for formation of each of the idiotypes studied at the chemical level. However, previous studies involving backcrosses of F(1)(Id(+) x Id(-)) mice to the Id(-) parental strain have never employed as the Id(-) parent one of several inbred strains shown by G. M. Edelman and P. D. Gottlieb [(1970) Proc. Natl. Acad. Sci. USA 67, 1191-1199] to express a V(L)-region polymorphism. Among backcrosses performed in the present study, one involved the A/J strain as the Id(+) parent and the PL/J strain, one of the several strains with and L chain polymorphism, as the Id(-) parent. Whereas in three other backcrosses performed, idiotype expression segregated with H chain allotype, in the backcross to PL/J, all mice producing the characteristic A/J Id(+) phenotype were A/J allotype(+), but not all A/J allotype(+) mice were Id(+). Typing of backcross progeny for the Ly-3 thymocyte alloantigens, controlled by a locus closely linked to that governing the L chain polymorphism (called the VK-1 locus), indicated that only the Ly-3 heterozygotes expressed the characteristic A/J Id(+) phenotype. Thus, all Id(+) backcross mice inherited both the H chain allotype and the Ly-3 locus (and closely linked L chain-related locus) from the Id(+) A/J strain. This suggests that, when strains with L chain polymorphisms are included in genetic studies of idiotype expression, segregation of loci governing L chain expression may be found to contribute to inheritance of the Id(+) character. In addition, these studies may be taken as further evidence that the V(L)-region repertoire of the PL/J strain may differ considerably from that of most other inbred strains of mice.