A new light-chain marker has been detected in normal mouse serum immunoglobulin light chains by gel isoelectric focusing. The marker (Ef2) involves the presence of two major and several minor bands in the normal light-chain IF profiles. Strains expressing the marker IF bands are designated Igk-Ef2a, whereas those lacking the bands are Igk-Ef2b. The majority of inbred strains are Igk-Ef2a. Strains found to be Igk-Ef2b are NZB/BlNJ, BDP/J, C58/J, I/LnJ, CE/J, and P/J. The strain distribution of the alleles differs from the distribution of alleles at the Ly-2 and Ly-3 loci, suggesting the new marker may represent a separate locus. Genetic studies have shown that Igk-Ef2 locus is closely linked to Igk-Ef1 and Hd loci on Chromosome 6, indicating that it is also closely linked to Ly-3. The relative importance of the bands controlled by the Igk-Ef2 locus suggests that the entire normal light-chain pool could be controlled by as few as 100 such loci.