The immune macroglobulin from the longnose gar, Lepisosteus osseus, has been purified and physicochemi-cally characterized. The macroglobulin was found to have a sedimentation coefficient of 13.9 S and a molecular weight of 610,000. There was a total of 4.97% carbohydrate with a residue distribution different from human macroglobins. The molecule was found to contain 82 disulfide bonds and dissociation into heavy (H) and light (L) chains could be observed following total reduction and alkylation in 7 m guanidine hydrochloride. The H and L chains could be separated by gel nitration and were found to have molecular weights of approximately 70,000 and 23,000, respectively. The macroglobulin was examined by electron microscopy and the molecule displayed a tetrameric structure. The amino acid composition was similar to mammalian IgM H and L chains. Although no free N-terminal amino acid was detected in the L chain, the N-terminal sequence of the first 4 residues of the H chain was found to be Asp-Ala-Val-Val. All of these structural data indicate a strong phylogenetic relationship among the immunoglobulins. © 1971, American Chemical Society. All rights reserved.