A gene encoding the lambda 5 light chain constant region was isolated from a genomic library from the SPE mouse strain (C lambda 5S). SPE is an inbred wild mouse strain belonging to the Mus 3 or Mus spretus group that has been genetically isolated from Mus 1 (the group to which laboratory mice belong) for a period of 1-3 million years. The sequence of the C lambda 5S gene shows strong homology to C lambda 5 of (C57BL/6J x DBA/2)F1 both in the coding region (98% identity) and in the 5'- and 3'-flanking regions (98 and 95% identity, respectively). Sequence comparison of C lambda 5 genes with C lambda 1 of BALB/c shows only few substitutions in the C lambda 5 coding regions and suggests that the three genes have a common ancestor. These data indicate that the C lambda 5 gene has evolved under strong selective pressure and probably encodes a functional gene product. The conservation of the C lambda 5 gene in various Mus species was observed by high stringency Southern blot analyses using a C lambda 5S probe on DNA sample from members of four different groups of wild mice. All the laboratory and wild mouse strains tested, including those with amplified sets of C lambda 1 and C lambda 2 hybridizing sequences, showed only single C lambda 5 hybridizing fragments. Little variation in size of restriction fragments detected with the C lambda 5 probe was seen in the different Mus species suggesting a high degree of conservation.(ABSTRACT TRUNCATED AT 250 WORDS)