The solid ceilular Lesions are :nore inclined to arise in follfcies whicn are similar to the normals in size. A site of cellular proliferation oqcurs in the follicular wall. As many as 50% of the cells forming a tiny mass of pe haps a dozen cells are labelled with the “H-TdR label showing that within the 4 hours|before sacrifice many of the cells were picking wp tritiated thymidine in preparatign for mitosis. Before the relatively small follicle is filled with cells, some ofthe cells bearing the “H-TdR label are fod outside of the basement membrane of the] follicle. These apparently rapidly proliferating cells appear to be spreading to fbrm a solid cellular mass. Some of these lesions are partly encapsulated but femnants of what appear to be partial encapsulation are seen in the tumor mass. Sp ead of the neo- les may be found plasm beyond the true thyroid capsule ultimately may be found. where the 34-TaR label is moderate, but at the point where the lesfion has breeched a thyroid capsule and spread into surrounding fat, the “H-TdR labellfing margin of cells in greatly intensified. in the advancin | Rats, particularly the Long-Evans strain as used by Lindsay Chaikoff and associates are prone to develop spontaneous neoplasms composed of fi or C-cells as animals age. The neoplasms form solid masses of ce rat may be confused with the solid lesions arising from the folli rats also show considerable hyperplasia of C-cells which may be a “neoplasms. In our early experience we could not distinguish be lesions and considered them all'Solid cellular’ We intially as wrongly) that C-cell tumors did not represent a significant mmbeq of the neoplasms in our irradiated rats because the solid cellular lesions were ve¥y uncammon among controls in our particular strain of Sprague-Dawley rats. The fajlure to be able to identify which solid cellular lesions were of C-cell origin and cell origin not only left our analysis of the incidence of the ich were of follia types in question but also weakened our hypothesis that most of the solid lesions were of follicular