1. Pancreatin or Pronase hydrolysis of partially purified heat-denatured (125)I-labelled thyroglobulin does not release all the iodoamino acids from peptide linkage. 2. The addition of thyroidal proteases is essential to obtain maximum hydrolysis of thyroglobulin. 3. Pronase plus a fresh non-radioactive thyroid homogenate gives the best hydrolysis results, but peptide ;mapping' shows that about 10% of the radioactivity remains in the form of peptides. 4. These peptides have approximately the same R(F) as the iodotyrosines on one-dimensional chromatograms developed in butan-1-ol-2n-acetic acid (1:1, v/v). 5. Peptic hydrolysis of thyroglobulin releases virtually all of the iodothyronines but little of the iodotyrosines, in contrast with the action of pancreatin. 6. Hydrolysis of thyroglobulin with pepsin followed by Pronase plus a source of thyroidal proteases is satisfactory, but the results are not superior to those obtained by using Pronase plus a source of thyroidal proteases.