The immunoglobulin of the hagfish, Eptatretus burgeri, one of the most primitive vertebrates extant, was isolated from the serum of non-immune normal adult hagfish in a pure form. Analysis of the immunoglobulin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing condition indicated that the immunoglobulin was composed of heavy (H) and light (L) chains. The mol. wt of the H-chain was 68,000, slightly smaller than that of the human mu-chain. The L-chain of the immunoglobulin appeared as 2 bands on SDS-PAGE, with mol. wts of 25,000 and 22,000. These findings were confirmed by gel filtration of reduced-alkylated immunoglobulin in 5 M guanidine-HCl. The H:L molar ratio of the immunoglobulin was roughly 1:1. Gel filtration of the immunoglobulin in non-dissociating buffer indicated that the mol. wt of the intact immunoglobulin was 150,000-160,000. Thus, the subunit chain composition of the immunoglobulin was assumed to be H2L2, identical with the fundamental structure of immunoglobulins. The instability of the hagfish immunoglobulin was ascertained by the fact that it dissociated into heterogeneous mol. wt components ranging from approx. 90,000 to 160,000 upon SDS-PAGE under non-reducing conditions. However, almost no free or monomeric H- or L-chains were dissociated from the immunoglobulin by this procedure and also by gel filtration in 5 M guanidine-HCl. Theses results indicated that the hagfish immunoglobulin is unusually labile in its tertiary structure but has disulfide binding between at least more than 2 subunit chains.