As an initial step towards the molecular analysis of the murine V kappa locus, a cosmid library from BALB/cJ mouse liver DNA was screened with probes representing 10 V kappa families. Of eight cosmids that were isolated from the initial screen, five contained a single restriction fragment that hybridized to the probes. Two cosmids contained two fragments that hybridized to the same probe, V kappa 4, indicating that some V kappa 4 gene segments are linked. One cosmid had two genes that belonged to different families, V kappa 1 and V kappa 9. The two gene segments were located within 12 kb of each other and lay in the same transcriptional orientation. Linkage of gene segments from the V kappa 1 and V kappa 9 families is consistent with a genetic map of the locus, and provides physical evidence for the first time that two genes from different families are closely linked in the murine kappa locus. Sequence analysis revealed that both genes are pseudogenes: the V kappa psi 1.7 gene segment has eight mutations, including termination codons, insertions, and deletions, and the V kappa psi 9B.8 gene segment has two mutations of an insertion and altered RNA splice site. Both genes have the potential to rearrange based on the sequence of their heptamer-nonamer motifs. The identification of pseudogenes raises the question of how many nonfunctional genes are present in the murine germline repertoire.