Our previous analysis of an immunoglobulin gene encoding the rabbit kappa chain of b4 allotype revealed that of the five J-like sequences in the J kappa cluster of this gene, only one, J2, appeared to be functional. This unusual ratio of J pseudogenes to functional J genes is unique among all J clusters of light and heavy chain genes of all species examined to date, including the cluster from the rabbit kappa 2 isotype, and must have consequences for diversity generation of b4 immunoglobulins. The fact that the only two known b5 J kappa sequences are different from the functional J2 of the b4 allotype prompted investigation of the b5 J kappa cluster to determine whether it resembled the b4 cluster, or the more typical mouse or human J kappa clusters. Our analysis of the b5 gene reveals a J kappa cluster strikingly similar to that of b4; apparent defects occur in all J sequences except J2. Although J2 is apparently functional, it differs from the J2 of the b4 locus by four nucleotide and three amino acid substitutions. The unusually high degree of sequence similarity previously observed between the b4 and b5 loci in the noncoding (vs the coding) regions extends through the newly sequenced DNA segment and remains an enigma.