The germ-line elements for the variable (V) and joining (J) regions of the kappa chains (V kappa-Ars, J kappa 1) that give rise to the productive allele for the A/J mouse light chains of Ars-A monoclonal antibodies (directed against the hapten p-azophenylarsonate) have been cloned and sequenced as well as a rearranged Ars-A light chain gene. Using the V kappa-Ars gene as a hybridization probe, we provide evidence that the V kappa 10 family is relatively small and that only one member gives rise to the productive allele in all Ars-A antibodies. An unusual feature of Ars-A light chains is that all contain an arginine at position 96, the V-J junctional position. We had shown by chain recombination that arginine-96 was essential for antigen binding. In the present study, an arginine codon was not found either at the 3' end of the V kappa-Ars gene segment or at the 5' end of J kappa 1. However, an arginine codon (CGG) can easily be generated by recombination between these two germ-line elements. Thus, we document that junctional diversity through intracodonic recombination can be crucial to the antibody binding function of the resulting molecule.