Using a cloned cDNA of a mouse immunoglobulin kappa light chain synthesized in a myeloma MOPC321 (V kappa-21 subgroup C) as a probe we could detect 13 germ line V kappa gene segments. 11 of these were isolated. Using a set of overlapping cloned segments, we showed that nine of these germ line V kappa genes are arranged in two linkage clusters and that they all have the same transcriptional orientation (11, 12, 22). These two clusters occupy 90 and 30 kb of chromosomal DNA and contain six and three V kappa's, respectively. We determined the complete nucleotide sequences of five germ line V kappa's and showed that three of them encode the prototype sequence of V kappa-21 subgroups B, C, and E. None of these five germ line V kappa's encodes the variant amino acid sequences of known V kappa-21 subgroups. We thus conclude that, as in the lambda 1 light chains, the variant V regions are encoded by gene segments derived by a few somatic mutations from the corresponding germ line DNA. Such somatic mutations are not restricted to sequences encoding the hypervariable regions: they also occur in sequences encoding framework regions.