We have cloned a cDNA encoding a kappa light chain of the b4 allotype. The clone, pB4D5, was derived from the allotype-specific mRNA of a rabbit-mouse hybridoma 12F2, which secretes a monoclonal rabbit b4 kappa chain. Nucleotide sequence analysis of the entire pB4D5 insert reveals complete variable (V) and constant (C) regions and portions of the signal peptide and 3' untranslated region. The translated pB4D5 region amino acid sequence matches the NH2-terminal sequence of the kappa chain secreted by 12F2 and is similar, though not identical, to several other b4 kappa sequences. A V region-specific probe isolated from pB4D5 hybridizes to more than 25 bands on a genomic Southern blot of b4 rabbit DNA and yields a similar pattern with DNA from other allotypes. Sequence comparisons between pB4D5, a cloned germ-line precursor of the pB4D5 joining (J) region, and published b4 amino acid sequences reveal a peculiar feature of rabbit kappa V-J recombination in which this system resembles mouse and human heavy chain recombination more than any known kappa system.