- 15 cell crigin and that C-cell neoplasms were not readily induced by The solid lesions were very infrequent in controls of the same age Radioimmmoassays for tran animals which had been issue because the titre was quite variable from animal to animal of interfollicular cell hyperplasia (found in rats) varied from many of which did not have a demonstrable solid cellular tumor. Wi to Dr. Hunter Heath of the Mayo Clinic for carrying out sample ass tonin for us. Dr. E.D. Williams of the University of Wales, Ca are grateful of rat calci, being an author on the medullary carcinona of man and the interfollicular ceil les; ons of rats, was called upon for assistance in the identification of the C-cell les: ons. He has point out to us by direct personal assistance the characteristic feature: of these lesions while reviewing many microscopic examples. The features are rather ¢asily recognized. Since the interfollicular cells normally lie between the foll: cles, their proliferation, as in hyperplasia of such cells, tends to spread the ft llicles apart. A neoplasm arising from such cells not only expands the interfollic lilar space in a local area, but appears to completely surround an occasional norm’ follicle. The expanding neoplasm becames lobulated with thin commective tissue margins and septae around the clusters of proliferating cells, a feature which Willia is has appropriate! called "packeting"’. Normal follicles often aretrapped in these 7 rowing lobules. Fortunately our procedure of double labeling with 13Li and SH-TaR Pith radioautograpt has not only shown increased mitotic activity in such spreading les ions, but has alsc distinguished a trapped notmal follicle (because of 131 I uptake in it) fram the tiny follicles sometimes seen in the undifferentiated solidcellular les: lions with an occasional primative follicle which does not pick up iodine. The uncertainty of distinguishing C-cell neoplasms fram other solid cellular lesions prampted us to hold up publication describing the high inc: dence following low doses of 131, of neoplasms This was the first of a series of 35 anuscripts submitt