Considerable progress has been made in the characterization of immunoglobulin genes from several lower vertebrate taxa. Isolation and identification of immunoglobulin genes in phylogenetically primitive species is based predominantly on heterologous crosshybridization. The unit, clustered organization of heavy chain segmental elements observed in the germline of the horned shark (Hinds and Litman, 1986) has also been found in another elasmobranch. Studies to determine whether the clustered organization is universal throughout the entire cartilaginous fish assemblage are ongoing. In contrast, the ray-finned (bony)fishes appear to possess a mammalian-type heavy chain gene organization. Additionally, immunoglobulin genes are being characterized in two relict fish species whose exact systematic relationships are unknown. Isolation of putative immunoglobulin genes from the phylogenetically- ancient hagfish is being attempted using a PCR-based approach. Other ongoing or future research efforts involve characterization of lower vertebrate light chain genes, heavy chain isotype evolution, and the divergence of the immunoglobulins and T-cell antigen receptors