irraciated cells to take up tritiated thymidine and thus prepare fbr cell division under the stimulus. lus is applied. This explains the failure of the thyroid to eflarge when the stir we showed by quan- In other earlier experiments under these contractgk itative histochemical measurements of DNA that the nuclei of an ofcasional cell con- tained much more than two times the diploid value, at which time the cell should have divided. We pointed out many years ago when we obtained the first] biopsies of thyroi treated with 1 307 and 131; that the large bizarre nuclear forms wilth excessive chromatin resembled neoplastic cells that might have malignant potential. primary basis for our original AEC Contract). (This was the The cummulative evadence increasingly _ supports the idea that under such conditions those altered cells, Jvyhich can and do synthesize DNA and occasionally complete the process of mitosis aye the source of neoplasms. It does appear that with heavier irradiation the numba of. cells retainin: a capacity for mitosis is reduced and thus the chance of a neoplasm developing is les We know from previous animal experiments, and suspect from olfervations on clini subjects, that there is a difference between the effect of a relatfively large dose of 131; which causes both impairment to the synthesis of thyroxine ard intrinsic nuclear damage (that precludes subsequent cell division) and the small dose which damages the nucleus but does not seriously impair thyroxine production. The problem has been to determine these respective doses in the experimental animal. There is a dose level of 131 I to the thyroid in man that results in a latent failure of the thyroid hormone production after all of the isotopq is gone and years have passed. (See report by Larson, et al including this author)] As we pointed out more than 20 years ago from the work under the earlier AEC contracts, the failure is attributable to intrinsic nuclear damage which does not destro} the cell nor its hormone production but does result in ultimate death of the cell When mitosis is attempted.