Serum Ig from the Pacific hagfish, Eptatretus stouti, was isolated by affinity chromatography using a specific mAb (H.45). Analysis of the approximately 210-kDa molecule by SDS-PAGE under reducing conditions revealed two H chains of approximately 77 kDa (H1) and approximately 70 kDa (H2) and L chains of approximately 30 kDa. H1 and H2 were shown to differ with respect to their peptide maps, amino-terminal amino acid sequences, and reactivity to the mAb H.45, suggesting that they represent discrete H chain isotypes. Two-dimensional nonreducing/reducing SDS-PAGE demonstrated that H and L chains were covalently linked predominantly as H-H-L and H-L configurations. Noncovalently bound L chains were also found. H-H-L complexes were shown to contain H1-H2 heterodimers of H chains in addition to H1-H1 homodimers.