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Four allelic forms of the immunoglobulin lambda light chain constant region (CΛ) genes are found in the caucasoid population and are recognised by the presence of Eco RI restriction fragments of sizes 8, 13, 18, and 23 kb hybridising with a CΛ probe. The less common allelic forms of the CΛ genes are marked by the 13, 18, and 23 kb fragments, and these may be misinterpreted as rearranged fragments derived from a monoclonal population of B-cells. The 13, 18, and 23 kb alleles result from the insertion of one. two, and three copies, respectively, of a 5.2 kb repeat unit that can be demonstrated by hybridising genomic DNA digested with Hind III with a CΛ probe. Using selected histologically and immunophenotypically well-defined cases, we assessed the usefulness of the 5.2 kb Hind III/CΛ fragment in distinguishing between rearrangements indicative of a B-cell lymphoma and germ-line configurations representing uncommon allelic forms of the CΛ genes. From these studies, we conclude that Hind III digestion should routinely be performed when using the CK probe to assess B-cell monoclonality. Identification of the 5.2 kb Hind III fragment obviates the need to examine DNA from normal peripheral blood granulocytes to exclude allelism of the CΛ genes in cases exhibiting the uncommon restriction fragments after Eco RI digestion.