Serum TPO concentrations were studied compared with the hTG levels in 47 patients suffering from different thyroid carcinomas (before and several times after the first, second or third radioiodine therapy respectively. An immunoluminometric coated tube assay was applied recognizing this enzyme (Brahms Diagnostica GmbH). Elevated TPO levels could be seen in 18/47 pts. All of them suffered from large postoperative remnants or metastases. An increase of TPO concentration was seen frequently at days 2 and 3 following the first or second radioidine application. Although the hTG response seems to be more sensitive, TPO may be helpful to reduce the number of false positive hTG findings.