Immunoreactivity of 125I- anti-Tg- McAb（18A1 and 18A11） with cells of normal thyroid and tumor was determined by cell binding assay. Results showed that the binding percentage found in differentiated throid carcinoma medullary thyroid carcimena

benign thyroid tumor

and other cells was >30%

8%-12%

<10% and <4% respectively. Binding percentage of 125I-McAb with differentiated thyroid carcinoma cells was significantly higher than that of normal thyroid and benign thyroid tumor. No binding with cells of malignant tumor coming from the other tissue was confirmed. Ratio of binding percentage on differentiated thyroid carcinoma with normal thyroid cells was also determined as 3.4-4.3（simple McAb）

and 5.0（"cocktail" McAb）. Therefore both 18A1 and 18A11 can be successfully used for radioimmu-noimaging of differentiated thyroid carcinoma and its metastatic sites.