This study was undertaken to determine if a lack of V(L) domain variability could explain, in part, the failure of Atlantic cod to respond to immunization with the production of specific antibodies. The variability of cod V(L) regions was studied in 33 cDNA and two genomic clones. The variability of the CDRs was estimated by the Shannon entropy method and compared with that in other species. It was found to be lowest in the little skate (Raja erinacea), higher in cod, and highest in Xenopus and mouse. While the variability of the CDRs is slightly lower in cod than in Xenopus and mouse, it is spread over broader areas of the amino acid sequence. The length of CDR1 and CDR3 in cod is equal to or exceeds that found in skate, Xenopus, chicken and mammals. Isoelectric points and hydrophobicity vary substantially among the studied Ig light chain domains. Thus, neither the length, nor the variability, nor the physicochemical properties (pI and hydrophobicity) of the L chain CDRs can explain the absence of antibody response to immunization in cod.