Studies have been carried out on reduced and alkylated 19 S bovine thyroglobulin to characterize naturally occurring, iodine-rich fragments. In this report, the purification and properties of a 30-kDa, hormone-enriched polypeptide (TgE) are described and compared to that of a previously reported 10-kDa fragment (TgF). The amino acid sequence of TgF was found to overlap with that of TgE. In spite of its larger size, TgE contains only a single hormone bearing site. Both the 10- and 30-kDa fragments are derived from the NH2-terminal end of the bovine thyroglobulin. These fragments contain the principal hormone-forming site at residue 5 of the thyroglobulin sequence and appear to be formed by cleavage of the parent polypeptide chain. The mechanism which generates these cleavages is not clear since the sequences surrounding the cleavage points which give rise to these peptides are quite different. These two fragments may be precursor and product in such a process. The amino acid sequence contained within TgE includes two putative sites for N-linked glycosylation. Since no glucosamine was observed and only small amounts of neutral sugar were detected, it appears that this part of the molecule is not extensively glycosylated.