Immunoglobulin (Ig) genes encode antibody molecules expressed exclu­ sively in the B-Iymphocyte lineage. The most immature identifiable cells of the B-cell lineage, pre-B cells, give rise to B cells that differentiate terminally into antibody secreting plasma cells. Circulating antibody mol­ ecules are comprised of two identical light (L) chains and two identical heavy (H) chains. The expression of Ig genes is strictly regulated during development of B lymphocytes: At the pre-B-cell stage only Jl chain genes are expressed, while L chains begin to be synthesized at the B-cell stage (1, 2). Ig genes are unusual in that they are arranged in separate subgenes. The limiting step required for correct transcription of Ig genes is the assembly of H genes from three subgenes, variable (V), diversity (D), and constant (C), and of L genes from V and C (reviewed in 3). Since each of the three Ig loci (H, K, and 11.) consists of multiple V, (D), and J subgenes, a large number of different H and L genes can be assembled by joining of different combinations of the subgenes (3). Somatic hypermutability contributes additional diversity (4, 5). Thus, it can be estimated that thousands of different H and L chains are produced by an individual. Since millions of different H-L combinations exist, B lymphocytes are extremely heterogenous with respect to the antibodies expressed and conse­ quently to the antigens recognized. Lymphoid organs are a mixture of such highly diversified B cells, so it is impossible to study the development or immune response of a monoclonal B-cell lineage in the normal mouse. Malignant cell lines, such as myelomas in animals or cultured myeloma,