childhood increased the incidence of thyroid adenaonas ani cancer. Follow-up studies on hyperthyroid patients treated with large doses of '311 aiso indicated thet thyroid adenomas and carcinonas were produced (Dobyns et al., 1974). The carcinogenic potential of 1311 was also noted in animal experiments. Goldberg and Chaikoff (1951) gave 400 uCi 1311 to 10 rats and found 2 thyroid tumors. Doniach (1963) summarized the results of several studies done in the mid 1950s and early 1960s, in which the thyroid carcinogenesis of 131] was evaluated and compared with that produced by x-irradiation. In that review, 1311 was considered to be 1/10 as effective as x-ray in the production of cancer, 1-e. 10,000 rads from 1311 would produce in rats the same level of effects as 1000 rads from x-ray. Differences in Radiobiologic Effectiveness. Observations of other thyroidal effects from diverse animal studies support the existence of differences in effectiveness of the two radiation exposures. McClellan et al. (1963), for example, compared the histopathologic changes tn irradiated thyroids of sheep ard estimated 1511 to be about 1/20 as effective as x-rays for the product ion of histologic effects. In mice, where the inhibition of goftrogenic stimulation “wes used as an index of radiation effect, 1311 was 1/4 to 1/2 as effective as x-rays (Walinder and Sjoden, 1971). Oata fram rats, using the Same model as a ee ee test for effects, suggest 1311 to be about 1/8 as effective as x-irradiation (Grieg et al., 1970). Another study in rats, using thyrofd tumorigenesis as the end-point, showed 13! to be about 1/2 to 9/10 as effective as x-rays in the production of adenomas, but of equal effectiveness for the product tor of carctnanas (Lee et al., 1982). Unfortunately, animals in that study wre isiledt ?