Tumour radioiodine concentration has been compared with serum thyroglobulin (Tg) and, in a few cases, with tumour complement of thyrotrophin receptors in patients with differentiated thyroid carcinoma. All tumours examined possessed TSH receptors. In most the complement was similar to that of normal thyroid tissue although all but one of the tumours had no detectable 131I concentration in vivo even with excess TSH stimulation. Elevated serum Tg (patient taking T4 in suppressive dose) was generally associated with tumours which had 131I concentrating function when stimulated by excess TSH. Some patients, however, had high serum Tg concentration but only low or indetectable tumour 131I uptake. We conclude that (a) measurement of tumour TSH receptor complement is unlikely to be useful in clinical management as tumours which do not significantly concentrate 131I in vivo may have a normal TSH receptor complement and (b) the capacity to secrete Tg is usually associated with 131I concentration but quantitatively the relationship varies considerably between tumours.