The present study reports the biochemical and immunohistochemical findings in the cytosol of a Hürthle cell carcinoma as compared with that of normal thyroid tissue. Sephadex G-200 chromatography of the extract derived from a Hürthle cell carcinoma and from normal thyroid tissue revealed three identical pools. Pool I consisted mainly of thyroglobulin (Tg), pool II corresponded to albumin, while pool III contained unidentified low molecular weight fragments which could not be studied further. Hürthle cell carcinoma, pool I, had a Tg content of 12.9 micrograms Tg/mg equivalent tissue and a 127I content of 5,6 mole/mole of Tg. Its sialic acid content was undetectable, however. In pool I of the normal thyroid gland, the respective values were 62.8 micrograms Tg/mg equivalent tissue, 21.3 +/mole 127I/mole Tg, and 15.4 mole sialic acid/mole Tg. The albumin contained in both pools II was shown to be ioidinated at the following levels: 0.025 mole 127I/mole albumin in Hürthle tumor pool II vs 1.28 mole 127I/mole albumin in normal thyroid pool II. Immunohistochemical studies confirmed the presence of Tg and albumin in the malignant Hürthle cells and acini and colloid. Thus, Hürthle cell carcinoma contained Tg and albumin. The Tg content was five times less compared with control tissue. Both proteins (Tg and albumin) were poorly iodinated in Hürthle carcinoma tissue, and the iodination of albumina seemed to be more severely impaired. The site of synthesis of both proteins could not be derived from the present studies.