We have cloned four lambda light chain constant region (C) genes from mouse embryo DNA. Each carries its own joining (J) segment approximately 1.3 kilobases to its 5' side. The four C genes occur in two clusters, 5' J3C3J1C13' and 5' J2C2J4C43', with C4 being a new C lambda gene. We have also shown that V lambda 1 is joined productively with C lambda 3 in a lambda 3-producing myeloma, and it is most likely that V lambda 1 and V lambda 2 are the only V lambda genes. Based on the analysis of the germ line and rearranged variable region (V) lambda genes in myelomas we argue that the V lambda 1 and V lambda 2 genes are at the 5' side of the C3C1 and C2C4 clusters, respectively. We propose that the two clusters arose by duplication. We also speculate on the role of J-associated DNA sequences in regulation of expression of the lambda subtypes.