We previously demonstrated that loci closely linked to the Ly-3 locus control the expression of distinct sets of light chains in normal mouse serum immunoglobulin. One of these loci, IgK-Ef2, was shown to control two major bands in normal light chain isoelectric focusing (IF) profiles. Strains possessing the marker bands were designated IgK-Ef2a. Screening of myeloma proteins from the strains BALB/c (IgK-Ef2a) and NZB (IgK-Ef2b) led to the identification of eight proteins in the BALB/c collection having light chains that cofocus precisely with the polymorphic IF bands observed in normal serum light chains. Partial sequence analysis of 3 of the light chains has shown that they are all identical in the first 30 positions, which indicates that they constitute a single variable region of the kappa light chain (VK) group (VK1). The frequency of occurrence of the group within the BALB/c myeloma collections (8 out of 277) suggests that the number of such groups may be closer to 50 than to 100. The finding supports an interpretation of the genetic polymorphism as being in part a result of the absence of genes related to VK1 in IgK-Ef2b strains of mice.