To define the polymorphism and extent of the mouse immunoglobulin kappa (Igk) gene complex, we have analyzed restriction-enzyme digested genomic DNA from 33 inbred strains of mice with labeled DNA probes corresponding to 16 Vk protein groups (1 of them previously undescribed) and the Jk/Ck region (V, variable; J, joining; C, constant). These probes detected between 1 and 25 distinct restriction enzyme fragments (REF) that appeared in up to eight polymorphic patterns, thus defining eight mouse Igk haplotypes. The investigated portion of the Vk repertoire was estimated to encompass between 60 and 120 discernable Vk gene-containing REFs. In contrast to mouse VH gene families, several Vk gene families defined by these probes appeared to overlap. This observation has implications for Vk gene analyses by nucleic acid hybridization and raises the possibility that the Vk gene complex is a continuum of related sequences.