Regulation of rabbit Ig C kappa 1 L chain gene expression is thought to be under allelic control. Four nominal allotypic variants designated b4, b5, b6, and b9 have been found to be co-dominantly expressed at the C kappa 1 gene locus. Latent allotypes are nonallelic and appear as unexpected and transitory molecules in the serum as well as on B cell surfaces. To determine the mechanism of latent kappa 1 allotype expression, rabbits were initially induced to produce latent b5 and b6 allotypes by infection with Trypanosoma brucei and RNA was extracted from the lymph nodes of these rabbits. The polymerase chain reaction using allele specific oligonucleotide primers was used to detect and amplify the mRNA encoding the latent b5 and b6 sequences. Nucleotide sequence analysis of the cloned products revealed that the latent b5 and b6 cDNA sequences were identical to their nominal allelic counterparts, b5 and b6, respectively. Thus, the identification of transcripts encoding latent b5 and b6 sequences rules out serologic artifacts and idiotypic mimicry as explanations of latent allotype expression.