To test the hypothesis that subtle radiation effects in thyroids can identified, a collaborative study was set up with Dr. Robert Conard of Broo National Laboratory. A large collection of thyroid tissues known to have to varying degrees of radiociodi-e radiation (including controls) was unknowns to this writer in order studies carried on under this com submi ~ test the experience gained from the ¢. The sources of microscopic slides consisted of the following:1)Tissues patients in the Marshallese group who were submitted to thyroidectomy and wip to have been exposed to some reasonable amounts of radioicdine fallout. 2) thyroid tissues obtained from children with nodular goiter in the Nevada-U fallout was known to exist in 1954, but the extent of radiation to the were known A group of area where children in this area is very doubtfully significant. 3) Thyroid tissues goiters removed from children in Safford, Arizona which served as a control Stusy. 4) Thyroids from survivors of the Japanese atomic bomb casualty pati radiation exposure is said to have been mostly external, and where fallout contain relatively little radioiodine. 5) A group of thyroids from patient: thyroidectomy far nodular goiter in several areas of the United States and could be determined had not received radiation exposure to the region of marks of identification were removed from the slides, except for sequential Multiple slides from the same individual were mixed in the collection. Al microscopic slides were assembled from a rather large mmber of hospitals, fhumbering. the techmiques of preparation with hematoxylin and eosin and mounting wre essentially the Bame so that no clue as to their source was recognizable. Primary emphasis for identification of radiation effect was placed on the preseme of the bizarre nuclear forms although, in a few instances where more extensipe destructim was produced, other eviderces of radiation effect were evident. As has bee out, studies carried on in this laboratory indicate that the bizarre nucle increased amounts of chromatin represent cell division in which there has bek up of DNA, but a failure to divide. A stimulus (such as that from mild i ment to thyroid function or a goitrogen) which will promte mitosis is inclined to bping out the bizarre nuclear forms. It appears from past work that the stimulus may be e subtle, so subtle that mild degrees of hypothyroidism which are not readily letectable ‘nuclei enlarge, the normal nucleus reaches a limit at which it divides into |The nucleus of the radiated cell sometimes contimes to enlarge and occasions lirregular in contour. The recognition of these unusually large nuclei is thd clue that the thyroid has been subjected to radiation. _Thus if the radiation exposure fhas been between ee enlarged muclei and the occasional nucleus that is significantly ‘larger than all of the others.