Qualitative properties of thyroglobulin (Tg) antibodies, in association with thyroid malignancy, suspected malignancy or other thyroid diseases, were studied in 177 patients. Retrospective clinical analysis revealed 137 patients to have thyroid carcinoma and 40 to have other thyroid diseases. Serum Tg was assayed by an immunoradiometric method. Thyroid microsomal (AMC) and Tg antibodies were measured by the particle agglutination method and the avidity of Tg antibodies by enzyme immunoassay (EIA). Assessment of the qualitative properties of Tg antibodies revealed that the high-avidity antibodies especially seem to bind circulating Tg. Thus any Tg value from a sample with detectable Tg antibodies is unreliable and should be interpreted with caution.