xture was used containing tyrosine, carrier-free radioactive (4311) potassifm iodide, ucose, glucose oxidase and phosphate buffer. It was observed that crude hofogenates sheep thyroid caused organic binding of 14% of the carrier—free 13lt added] to the stem. In contrast, purified mitochondrial preparations of soluble enzyme (brepared sonification of mitochondria) caused organic binding of 36.7% of the availpble 1311. The icdinating properties of a variety of homogenates and soluble mitochbndrial zyme preparations from different tissues were then studied. Only v small. amounts tyrosine were iodinated by mitochondrial preparations from the small intestine and leen, but homogenates as well as soluble mitochondrial preparations from luhg Splayed a surprising capacity to iodinate tyrosine. The surprising observafion has en repeated in 4 separate experiments. Chromatography revealed that some of the dination produced by preparations fron thyroid tissue appeared to be bound fo protein, me to tyrosine. It is assumed that the protein had been introduced as intrfmitochondrial otein that had been freed by sonification. When tyrosine was added to the buspension % of the 1311 was bound to it while only 5.4% was bound to available proteih. When no rosine was added to the preparation 48% of the i3ly was bourd to protein. [lectrophoretic terns of the reaction mixture sampled after the incubation had taken placej showed wt the radioactivity was primarily in the position of albumin if tyrosine h&d been ded but in the position of gamma gloculin if tyrosine had not been added. Finally some experiments uco dye method for assaying Bll amounts of tyrosine but mg showed strong peroxidase rTosine as well. were performed to compare the radioiodine methofi and the peroxidase. Spleen and small intestine iodinathkd very very effectively oxidized the leuco dye. Thyroid ard activity by the leuco dye method and caused iodination of Thus, it appeared that. the peroxidase activity measured by the leuco re method was not necessarily identical with the peroxidase activity responsible for dination of tyrosine. The work on the leuco dye method of assay for peroxifase in roid tissue has not been published. The assay method based on the iodinatgion of mMosine was a method which was described before this phase of the work was Pompleted. : was hoped that the methods would be useful in radiation studies but has nok yet been » used, primarily because the interrelationships of reactions being observed in the dination of protein and tyrosine are not: understood. ‘ray Radiation Effect on the Thyroid Another part of the study of the effect of irradiation on the thyroid hap been search for thyroid masses in 200 patients previously given x-ray therapy tq the neck mw tuberculous cervical lymphadenitis. All of these individuals had been trpated in ve tuberculosis clinic of this hospital befare 1950 by the same radiologist.J Most of ese individuals were children or young adults when treated. Ail records ag to the wunt and fields of radiation were available. A total of 67 patients have Heen traced Wd called back for examination of the thyroid by us. Twelve of these were fpund to ive discrete thyroid masses. Nine of these discrete thyroid masses have beqh removed ‘th the following findings: 3 1 1 4 carcinomas Hurthle cell tumor with capsular invasion Hurthle cell tumor classified as benign with follicular adenomas