Human immunoglobulin light-chain genes become rearranged in an ordered fashion during pre-B-cell development such that rearrangement generally occurs in κ genes before λ genes (refs 1,2). This ordered process includes an unanticipated deletion of the constant κ (C_κ) gene and κ enhancer sequence which precedes λ rearrangement^1–4, and the site of this deletional recombination was located 3′ to the joining ( J_κ) segments in 75% of cases studied. We have now characterized the recombinational element responsible for this event on three separate alleles and found them to be identical. This κ-deleting element recombined site-specifically with a palindromic signal (CACAGTG) located in the J_κ–C_κ intron. All losses of C_κ genes in other human B cells were mediated by this determinant, including the 25% of instances when this element recombined with sequences 5′ to J_κ. In contrast, the κ-deleting element remained in its germline form on all successful κ-producing alleles. Moreover, κ loss is an evolutionary conserved event, as the κ-deleting element appears to be the human homologue of the murine RS sequence⁵. Our results suggest that this element may help ensure isotypic and allelic exclusion of light chains and may be involved in the ordered use of human light-chain genes.