The dissociation of thyroid 27 S iodoprotein by sodium dodecyl sulfate (SDS) and by succinic anhydride was investigated by means of ultracentrifugation and polyacrylamide gel electrophoresis. The iodoprotein obtained from either a human or hog was dissociated into three kinds of subunits (S-19, S-17 and S-12) by SDS treatment. At increased concentrations of SDS, the S-12 subunit was predominant among the dissociation products. The succinylation of 27 S iodoprotein showed essentially the same dissociation pattern as in the case of SDS treatment. This dissociation products of the protein preparations of different animals were qualitatively the same as those of thyroglobulin of the respective animals, confirming the hypothesis that 27 S iodoprotein was composed of two molecules of thyroglobulin. However, the extent of dissociation of 27 S iodoprotein measured by S-12 formation showed higher resistancy of the protein to the dissociating agents than that of thyroglobulin. The contents of sialic acid and hexose as well as iodoamino acids of 27 S iodoprotein were found to be the same as, or not far from, those of thyroglobulin; The dissociability and chemical composition of 27 S iodoprotein was discussed with reference to the subunit structure of the protein.